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

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