Investigate multiple `FirebaseUI/Provider` subspecs

[asciimike]

Investigate splitting FirebaseUI into more subspecs for specific auth providers. This requires several things:

1. Create the subspecs to only pull in certain source files and resources
2. Forward declare classes in header files where necessary
3. Perform runtime checks in shared auth implementations to guard against weakly linked libs

The start of a subspec is here:

s.subspec 'Auth' do |auth|
    auth.dependency "FirebaseUI/Facebook"
    auth.dependency "FirebaseUI/Google"
    auth.dependency "FirebaseUI/Twitter"
    auth.dependency "FirebaseUI/Password"
  end

s.subspec 'AuthHelper' do |helper|
  helper.source_files = "FirebaseUI/Auth/**/{FirebaseAppDelegate,FirebaseLoginViewController,FirebaseAuthConstants,FirebaseAuthDelegate,FirebaseAuthProvider,FirebaseLoginButton}.{h,m}"
  helper.resources = "FirebaseUI/Auth/Resources/*"
end

s.subspec 'Facebook' do |facebook|
  facebook.source_files = "FirebaseUI/Auth/**/FirebaseFacebookAuthProvider.{h,m}"
  facebook.dependency "FirebaseUI/AuthHelper"
  facebook.dependency "FBSDKCoreKit"
  facebook.dependency "FBSDKLoginKit"
end

s.subspec 'Google' do |google|
  google.source_files = "FirebaseUI/Auth/**/FirebaseGoogleAuthProvider.{h,m}"
  google.dependency "FirebaseUI/AuthHelper"
  google.dependency "Google/SignIn"
end

s.subspec 'Twitter' do |twitter|
  twitter.source_files = "FirebaseUI/Auth/**/{FirebaseTwitterAuthProvider,TwitterAuthDelegate}.{h,m}"
  twitter.dependency "FirebaseUI/AuthHelper"
  twitter.ios.framework = "Accounts"
end

s.subspec 'Password' do |password|
  password.source_files = "FirebaseUI/Auth/**/FirebasePasswordAuthProvider.{h,m}"
  password.dependency "FirebaseUI/AuthHelper"
end

[asciimike]

Ok, fixed as of 0.3.2 :)