Add support for Firefox JS testing

This runs in it's current state (make sure you have slimerjs and casperjs installed), but quite a few tests are failing.  I don't think it feasible to get the tests working for 2.0 final, but it's something to keep in mind for a later release.
For more info:
See line near the bottom about "exit code": http://docs.slimerjs.org/current/differences-with-phantomjs.html#in-the-api
Reported as a Casper bug here: https://github.com/n1k0/casperjs/issues/904