added method to set thumb image

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

Sets an image for the thumb. It only supports static images
Closes https://github.com/facebook/react-native/pull/3849

Reviewed By: svcscm

Differential Revision: D2665699

Pulled By: nicklockwood

fb-gh-sync-id: 3a767e43170074e2419067d5c8eae61668ebb5e9

Libraries/Components/SliderIOS/SliderIOS.ios.js

@@ -83,6 +83,11 @@ var SliderIOS = React.createClass({
      */
     trackImage: Image.propTypes.source,
 
+    /**
+     * Sets an image for the thumb. It only supports static images.
+     */
+    thumbImage: Image.propTypes.source,
+
     /**
      * Callback continuously called while the user is dragging the slider.
      */
@@ -114,7 +119,7 @@ var SliderIOS = React.createClass({
     });
 
     let {style, ...props} = this.props;
-    style = [styles.slider, this.props.style];
+    style = [styles.slider, style];
 
     return (
       <RCTSlider