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.
Allow FileLinks to not recursive into subdirectories
Remove unneeded modules, and added docstring info on recursive parameter
Add None as a widget visible value to allow for visiblity: hidden

Now, visible can have three values:
  * True - show the widget
  * False - hide the widget and collapse the empty space
  * None - hide the widget and show the empty space
Merge remote-tracking branch 'public-upstream/master' into links-rebase

Conflicts:
	examples/Interactive Widgets/Widget Events.ipynb
editor progress

- load/store codemirror config
- fill out menus a bit:
  - Edit/find,replace
  - View/toggle line numbers
track dirty state in editor for onbeforeunload
basic LESS for editor
allow selecting codemirror keymap in menu
link to /edit/ on files
add File/New in editor
fixup
fixup keyMap
setting an option to null sets the default in CodeMirror

matching the unset behavior in config