Use ref instead of findDOMNode.

Fixes part of langateam#17.

See jsx-eslint/eslint-plugin-react#678

lib/KendoWidgetMixin.js

@@ -43,7 +43,7 @@ var KendoWidgetMixin = function (widget) {
     componentDidMount: function () {
       if (this.props.debug) console.log('kendo widget mounting... ', widget);
 
-      this.elem = ReactDOM.findDOMNode(this);
+      this.elem = this.refs.elem;
       this.$elem = $(this.elem);
       this.$widget = mountKendoWidget(this, widget);
 
@@ -115,6 +115,7 @@ var KendoWidgetMixin = function (widget) {
      */
     render: function () {
       var other = _.omit(this.props, [ 'options', 'children', 'tag' ]);
+      other.ref = "elem";
       return React.DOM[this.props.tag](other, this.props.children);
     }
   };