Drag n drop box style dragging feature

This pull request adds a new drag and drop style to jqtree, my collaborator (@kennykaye) and I refer to this as box style dragging.  We think this change makes jqtree even more versatile and adds support for a dragging style that is popular elsewhere ( e.g. this [angular plugin](http://jimliu.github.io/angular-ui-tree/)).

I have created an example page which shows this feature in action, I wasn't quite sure how to build the `tmpl` files so I just did it manually.

 I tried to extend the existing code and keep this new handler style confined to a single coffescript file. I had to slightly modify the existing drag and drop class.

![gif-2](https://cloud.githubusercontent.com/assets/1691120/3436325/7f6905dc-009f-11e4-9bd3-db7aa2681ac1.gif)