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.
I removed the print statement, and now the output has gone from:
```
Test group: js/tree
Running tests with notebook directory '/tmp/tmp0q8jeh'
Test file: /home/jon/ipython/IPython/html/tests/tree/dashboard_nav.js           
PASS Testing dashboard link: http://127.0.0.1:8888/tree/sub ∂ir1
PASS Testing dashboard link: http://127.0.0.1:8888/tree/sub ∂ir1/sub ∂ir 1a
PASS Testing dashboard link: http://127.0.0.1:8888/tree/sub ∂ir2
PASS Testing dashboard link: http://127.0.0.1:8888/tree/sub ∂ir2/sub ∂ir 1b
PASS 4 tests executed in 2.17s, 4 passed, 0 failed, 0 dubious, 0 skipped.       

```

to a much less verbose form, where you can't tell which tests are passing and failing:
```
Test group: js/tree
Running tests with notebook directory '/tmp/tmph8z88w'

Test group: js/base
Running tests with notebook directory '/tmp/tmpo86l4j'
``