Revert "Remove context interface from Connector (PyMySQL#295)"

This reverts commit c64915b.

Author: Silviu Surcica

MySQLdb/connections.py

@@ -216,6 +216,20 @@ def query(self, query):
             query = bytes(query)
         _mysql.connection.query(self, query)
 
+    def __enter__(self):
+        from warnings import warn
+        warn("context interface will be changed.  Use explicit conn.commit() or conn.rollback().",
+             DeprecationWarning, 2)
+        if self.get_autocommit():
+            self.query("BEGIN")
+        return self.cursor()
+
+    def __exit__(self, exc, value, tb):
+        if exc:
+            self.rollback()
+        else:
+            self.commit()
+
     def _bytes_literal(self, bs):
         assert isinstance(bs, (bytes, bytearray))
         x = self.string_literal(bs)  # x is escaped and quoted bytes