Feature auto margin

Summary:
Even so I know there are some opinions against ```margin: 0 auto``` it's still part of the spec: https://www.w3.org/TR/css-flexbox-1/#auto-margins and pretty usefull if you have to position via ```justify-content```.

This PR adds an implementation for that.

It adds an additonal ```YGUnitAuto``` and margins got ```YGNodeStyleSetMarginAuto``` functions as well.
Closes #357

Reviewed By: astreet

Differential Revision: D4501142

Pulled By: emilsjolander

fbshipit-source-id: 86519f8632496f46e78a7c9dbc5b21e212e3e0c7

Author: woehrl01

csharp/Facebook.Yoga/Native.cs

@@ -196,6 +196,9 @@ public static extern void YGNodeSetBaselineFunc(
         [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
         public static extern void YGNodeStyleSetFlexBasisPercent(YGNodeHandle node, float flexBasis);
 
+        [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
+        public static extern void YGNodeStyleSetFlexBasisAuto(YGNodeHandle node);
+
         [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
         public static extern YogaValue YGNodeStyleGetFlexBasis(YGNodeHandle node);
 
@@ -205,6 +208,9 @@ public static extern void YGNodeSetBaselineFunc(
         [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
         public static extern void YGNodeStyleSetWidthPercent(YGNodeHandle node, float width);
 
+        [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
+        public static extern void YGNodeStyleSetWidthAuto(YGNodeHandle node);
+
         [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
         public static extern YogaValue YGNodeStyleGetWidth(YGNodeHandle node);
 
@@ -213,6 +219,9 @@ public static extern void YGNodeSetBaselineFunc(
 
         [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
         public static extern void YGNodeStyleSetHeightPercent(YGNodeHandle node, float height);
+        
+        [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
+        public static extern void YGNodeStyleSetHeightAuto(YGNodeHandle node);
 
         [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
         public static extern YogaValue YGNodeStyleGetHeight(YGNodeHandle node);
@@ -278,6 +287,9 @@ public static extern void YGNodeSetBaselineFunc(
         [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
         public static extern void YGNodeStyleSetMarginPercent(YGNodeHandle node, YogaEdge edge, float margin);
 
+        [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
+        public static extern void YGNodeStyleSetMarginAuto(YGNodeHandle node, YogaEdge edge);
+
         [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
         public static extern YogaValue YGNodeStyleGetMargin(YGNodeHandle node, YogaEdge edge);
 

csharp/Facebook.Yoga/YogaNode.Spacing.cs

@@ -226,6 +226,10 @@ private void SetStyleMargin(YogaEdge edge, YogaValue value)
             {
                 Native.YGNodeStyleSetMarginPercent(_ygNode, edge, value.Value);
             }
+            else if (value.Unit == YogaUnit.Auto)
+            {
+                Native.YGNodeStyleSetMarginAuto(_ygNode, edge);
+            }
             else
             {
                 Native.YGNodeStyleSetMargin(_ygNode, edge, value.Value);

csharp/Facebook.Yoga/YogaNode.cs

@@ -270,6 +270,10 @@ public YogaValue FlexBasis
                 {
                     Native.YGNodeStyleSetFlexBasisPercent(_ygNode, value.Value);
                 }
+                else if (value.Unit == YogaUnit.Auto)
+                {
+                    Native.YGNodeStyleSetFlexBasisAuto(_ygNode);
+                }
                 else
                 {
                     Native.YGNodeStyleSetFlexBasis(_ygNode, value.Value);
@@ -290,6 +294,10 @@ public YogaValue Width
                 {
                     Native.YGNodeStyleSetWidthPercent(_ygNode, value.Value);
                 }
+                else if (value.Unit == YogaUnit.Auto)
+                {
+                    Native.YGNodeStyleSetWidthAuto(_ygNode);
+                }
                 else
                 {
                     Native.YGNodeStyleSetWidth(_ygNode, value.Value);
@@ -310,6 +318,10 @@ public YogaValue Height
                 {
                     Native.YGNodeStyleSetHeightPercent(_ygNode, value.Value);
                 }
+                else if (value.Unit == YogaUnit.Auto)
+                {
+                    Native.YGNodeStyleSetHeightAuto(_ygNode);
+                }
                 else
                 {
                     Native.YGNodeStyleSetHeight(_ygNode, value.Value);

csharp/Facebook.Yoga/YogaUnit.cs

@@ -14,5 +14,6 @@ public enum YogaUnit
         Undefined,
         Pixel,
         Percent,
+        Auto,
     }
 }

csharp/Facebook.Yoga/YogaValue.cs

@@ -70,6 +70,15 @@ public static YogaValue Undefined()
             };
         }
 
+        public static YogaValue Auto()
+        {
+            return new YogaValue
+            {
+                value = 0f,
+                unit = YogaUnit.Auto
+            };
+        }
+
         public static YogaValue Percent(float value)
         {
             return new YogaValue