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/MapView/MapView.js

@@ -11,6 +11,7 @@
  */
 'use strict';
 
+const ColorPropType = require('ColorPropType');
 const EdgeInsetsPropType = require('EdgeInsetsPropType');
 const Image = require('Image');
 const NativeMethodsMixin = require('NativeMethodsMixin');
@@ -166,10 +167,7 @@ const MapView = React.createClass({
        * are supported for regular pins. For custom pin images, any tintColor
        * value is supported on all iOS versions.
        */
-      tintColor: React.PropTypes.oneOfType([
-        React.PropTypes.string,
-        React.PropTypes.number
-      ]),
+      tintColor: ColorPropType,
 
       /**
        * Custom pin image. This must be a static image resource inside the app.
@@ -213,14 +211,8 @@ const MapView = React.createClass({
        * Line attributes
        */
       lineWidth: React.PropTypes.number,
-      strokeColor: React.PropTypes.oneOfType([
-        React.PropTypes.string,
-        React.PropTypes.number
-      ]),
-      fillColor: React.PropTypes.oneOfType([
-        React.PropTypes.string,
-        React.PropTypes.number
-      ]),
+      strokeColor: ColorPropType,
+      fillColor: ColorPropType,
 
       /**
        * Overlay id