Drag handles / drop target data

This PR is two fold:

1) Allow for a drag handle (ng-drag-handle) to be specified as a child element attribute so only a specific element can be used for dragging.
2) Allow for ng-drop to be specified as arbitrary object data that will be passed to the drop success callback as a third parameter.

Both of these have proved useful for me on my current project -- I imagine others might be interested in the functionality as well.