update dragging className

Author: 淘小杰

lib/draggable.js

@@ -438,7 +438,7 @@ module.exports = React.createClass({
 
 		var className = CX({
 			'react-draggable': true,
-			'react-dragging': this.state.dragging
+			'react-draggable-dragging': this.state.dragging
 		});
 		// Reuse the child provided
 		// This makes it flexible to use whatever element is wanted (div, ul, etc)