Add a new mode: Run All (skip exceptions)

This is a prototype implementation of a mode discussed in #1977 to run all the cells in a notebook and skip over any exceptions.

I am new to Javascript and the IPython Notebook code, so this may not be the correct approach, but I would be interested in initial feedback (or suggestions for other approaches).

cc @fperez @minrk

Remaining to-dos:

* [ ] Documentation
* [ ] Changelog
* [ ] Tests (how are tests implemented for the notebook?)