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.
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
Make a copy of the input list so that changes to the children list don't automagically change our reference list
tweak documentation for viewlists
Basic infrastructure for new texteditor component
Loading a file works
Saving files works
Refactor editor into Editor class
Enable codemirror search/replace shortcuts