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/Touchable/TouchableHighlight.js

@@ -13,6 +13,7 @@
 
 // Note (avik): add @flow when Flow supports spread properties in propTypes
 
+var ColorPropType = require('ColorPropType');
 var NativeMethodsMixin = require('NativeMethodsMixin');
 var React = require('React');
 var ReactNativeViewAttributes = require('ReactNativeViewAttributes');
@@ -76,7 +77,7 @@ var TouchableHighlight = React.createClass({
      * The color of the underlay that will show through when the touch is
      * active.
      */
-    underlayColor: React.PropTypes.string,
+    underlayColor: ColorPropType,
     style: View.propTypes.style,
     /**
      * Called immediately after the underlay is shown