added a property to set a track image to slider ios

Summary: this change will allow the slider to have different track images.

Sets an image for the sliderIOS's track. It only supports images that are included as assets.
Closes https://github.com/facebook/react-native/pull/3850

Reviewed By: svcscm

Differential Revision: D2659680

Pulled By: nicklockwood

fb-gh-sync-id: faf6ddea1077b081c1fc05f8f110b669cef9902c

Libraries/Components/SliderIOS/SliderIOS.ios.js

@@ -11,6 +11,7 @@
  */
 'use strict';
 
+var Image = require('Image');
 var NativeMethodsMixin = require('NativeMethodsMixin');
 var PropTypes = require('ReactPropTypes');
 var React = require('React');
@@ -76,6 +77,11 @@ var SliderIOS = React.createClass({
      */
     disabled: PropTypes.bool,
 
+   /**
+     * Sets an image for the track. It only supports images that are included as assets
+     */
+    trackImage: Image.propTypes.source,
+
     /**
      * Callback continuously called while the user is dragging the slider.
      */