Adding Link widget
Adding directional link widget
Fix whitespace
Updated example notebook
Load link widget javascript
Load the link models
on("destroy",...) -> once("destroy",...) so we don't keep a reference to it, preventing gc

Thanks to Sylvain Corlay for the suggestion.
Merge remote-tracking branch 'public-upstream/master' into links-rebase

Conflicts:
	examples/Interactive Widgets/Widget Events.ipynb
Fix a bug in using promises with comms: this -> that
Add the unlink method to javascript links to maintain compatibility with traitlet links
Merge pull request #6454 from jasongrout/links

Javascript link