Added ColorPropType

Summary:
Problem: https://github.com/facebook/react-native/issues/4708

Solution: Added a ColorPropType that validates the color used by the dev

Notes:
1) I'm working a Win8.1 machine and couldn't build the react-native using the github repo. As soon as I figure that out, I'll probably figure how to run the tests and how to add some for this feature.
2) It's my first pull request. Be gentle :)
Closes https://github.com/facebook/react-native/pull/4866

Reviewed By: bestander, svcscm

Differential Revision: D2783672

Pulled By: nicklockwood

fb-gh-sync-id: ca22aa3c0999188075681b5d20fff0631496e238

Libraries/Components/SwitchIOS/SwitchIOS.ios.js

@@ -13,6 +13,7 @@
  */
 'use strict';
 
+var ColorPropType = require('ColorPropType');
 var NativeMethodsMixin = require('NativeMethodsMixin');
 var PropTypes = require('ReactPropTypes');
 var React = require('React');
@@ -62,17 +63,17 @@ var SwitchIOS = React.createClass({
     /**
      * Background color when the switch is turned on.
      */
-    onTintColor: PropTypes.string,
+    onTintColor: ColorPropType,
 
     /**
      * Background color for the switch round button.
      */
-    thumbTintColor: PropTypes.string,
+    thumbTintColor: ColorPropType,
 
     /**
      * Background color when the switch is turned off.
      */
-    tintColor: PropTypes.string,
+    tintColor: ColorPropType,
   },
 
   getDefaultProps: function(): DefaultProps {