First commit - a working Drag'n'Drop tree (bugged on IE9 for now)

TODO:
  - allow the user to interact with the components (before-drop / after-drop...)
  - IE9 support (native DnD work only with link)
  - Extract a b-droppable components
  - Allow drop of external node
Sort element, b-tree style, no IE9 support

Complexify exemple (ability to use whatever you want in a li element) + collapsible.
TODO:
Event propagation: send pre/post drop event, cancelable
Collapsible: find a way to attach the icon wherever you want