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.
Add notion of action that differs from  shortcuts.

This decouple the notion of shortcut from the notion of executed "action"
This allow the shortcuts manager to be purely describe as data, and the
same action to be later refered to either from the shortcut, from a toolbar
button or a menu.

This also implement a more complete keyboard shortcut handler which is
able ton interpete sequences like `Cmd-X,Meta-v` By storing the
shortcuts in a tree.
remove debug
add test warning shadow
nbconvert --to script

Use extension and mime type from NB metadata
'Download as' script
Update docs about kernel_info_reply
Fix asyncy nbconvert to download
Refactor the do_diff and manual child view lists into a separate ViewList object
allow unrecognized outputs and cell types in notebooks from the future

in nbformat schema / validation
Handle unrecognized outputs and cells from the future
Make error messages more consistent
Make state_change initialization more conventional
Missing semicolon