add "drag-start-class" for "ui-draggable", do some clean up on "$destroy"
correct var name "attr" to "attrs"
deep copy check for drag data change
Changes:
  - Change scope eventname with "ngDragDrop" namespace

Fixes:
  - Unbind "dragover" and "drop" events after drop successful
Changes:
  - Make draggable configuration accept angular expression
  - Deal with error message of IE's dragDrop() method in modern browsers
Changes:
 - "drag-enter-class" changed to "drag-accept-class". Activates when the drop zone is available.
 - Add "drag-over-class". Triggered when the element entering the drop zone.
Merge branch 'develop'