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/TabBarIOS/TabBarIOS.ios.js

@@ -11,6 +11,7 @@
  */
 'use strict';
 
+var ColorPropType = require('ColorPropType');
 var React = require('React');
 var StyleSheet = require('StyleSheet');
 var TabBarItemIOS = require('TabBarItemIOS');
@@ -29,11 +30,11 @@ var TabBarIOS = React.createClass({
     /**
      * Color of the currently selected tab icon
      */
-    tintColor: React.PropTypes.string,
+    tintColor: ColorPropType,
     /**
      * Background color of the tab bar
      */
-    barTintColor: React.PropTypes.string,
+    barTintColor: ColorPropType,
     /**
      * A Boolean value that indicates whether the tab bar is translucent
      */