initial require implementation
Get IPython widgets working with the cell server

Requires IPython commit b84b245b51a5b6d4e13a41ba79d773deaea59d63 or equivalent from the widget pull request or my branch of it (see https://github.com/ipython/ipython/pull/4374)
Fix cell server to work with updates to IPython

Works with commit b84b245b51a5b6d4e13a41ba79d773deaea59d63 on my branch.