Migrate tests to using jest

[ethanfrey]

This is preferred for testing react applications, especially with the virtual dom.

The workflow is not too different from mocha-chai and it should not be too hard to port all existing tests (which don't touch ui).

The downside (and why I didn't do this at first), was that they don't run in a browser, but everyone seems to believe that testing the virtual dom is good enough and better error messages.

This will make #6 obsolete