Add PhantomJS runner

This pull request adds a very basic runner for getting the unit tests to run in PhantomJS. The idea is that we can run this in Travis along with the node tests, and feel safer that the tests will stay green in browser environments also.

You can run it with

```
$ test/phantom/run.sh
```

One further improvement would be to silence the output of the python SimpleHTTPServer (or use another webserver to serve the files to PhantomJS). For now it works

##### TODO
- [x] Stop leaking a python process after each run
- [x] Fix failing tests
- [x] Update README.md with details on how to run tests in PhantomJS