add dragStart type

angular-ui-tree/angular-ui-tree-tests.ts

@@ -78,5 +78,6 @@ var droppedCallback: AngularUITree.IDroppedCallback = (eventInfo: AngularUITree.
 
 var callbacks: AngularUITree.ICallbacks = {
     accept: acceptCallback,
+    dragStart: droppedCallback,
     dropped: droppedCallback
 };

angular-ui-tree/angular-ui-tree.d.ts

@@ -54,6 +54,7 @@ declare module AngularUITree {
 
     interface ICallbacks {
         accept: IAcceptCallback;
+        dragStart: IDroppedCallback;
         dropped: IDroppedCallback;
     }