Fix react resizable class name

Author: zillding

examples/1.html

@@ -10,7 +10,6 @@
         background: #eee;
       }
       .box {
-        position: relative;
         background: #ccc;
         border: 1px solid black;
         text-align: center;

lib/Resizable.jsx

@@ -91,12 +91,16 @@ export default class Resizable extends React.Component {
 
   render() {
     let p = this.props;
+    let className = p.className ?
+      `${p.className} react-resizable`:
+      'react-resizable'
 
     // What we're doing here is getting the child of this element, and cloning it with this element's props.
     // We are then defining its children as:
     // Its original children (resizable's child's children), and
     // A draggable handle.
     return cloneElement(p.children, assign({}, p, {
+      className,
       children: [
         p.children.props.children,
         <DraggableCore