Add support for selenium UI tests.

UI tests must be in a file matching the pattern `uitest*.py`.
They can be run by using the `selenium` management command.

The tests are not headless, so they require a local X server to be running on the host (Xvfb is not on the Vagrant VM).

We intend to use Sauce Labs for CI, so this shouldn't be an issue for CI builds.
@jwalgran Unless you have issues with it I'm going to keep this in the `manage.py` file.

I tried putting this in `manage.sh`, but it gets tricky when you want to pass in other arguments to test (like specific tests to run) and I'd like running `manage.py` from inside the VM to work.