add btf-dragon-base and btf-dragon-container

[johlrich]

Adds optional ability to separate the base directive from the container holding the draggable elements.

In this example you can drop on the image, but only drag from the item list:

<div btf-dragon="thing in things" btf-dragon-base>
  <img src="http://placekitten.com/200/200">
  <div btf-dragon-container>{{thing}}</div>
</div>

Thoughts?

[cesarandreu]

Merged your changes on my own version, worked perfectly. Thanks!

This should be added to the master one.