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/ProgressBarAndroid/ProgressBarAndroid.android.js

@@ -15,6 +15,7 @@ var React = require('React');
 var ReactPropTypes = require('ReactPropTypes');
 var ReactNativeViewAttributes = require('ReactNativeViewAttributes');
 var View = require('View');
+var ColorPropType = require('ColorPropType');
 
 var requireNativeComponent = require('requireNativeComponent');
 
@@ -88,7 +89,7 @@ var ProgressBarAndroid = React.createClass({
     /**
      * Color of the progress bar.
      */
-    color: ReactPropTypes.string,
+    color: ColorPropType,
     /**
      * Used to locate this view in end-to-end tests.
      */