Add ng-drag-enter and ng-drag-leave attributes and events

[cdauth]

This patch adds the ng-drag-enter and ng-drag-leave attributes to the ng-drop directive. They are executed when a dragging element enters and leaves the drop element.

It also adds the events draggable:dragEnter and draggable:dragLeave events.

[cdauth]

I rebased on master to resolve merge conflicts.

Any reason why this is not being merged?