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.
I have no idea.  I don't want to type `--settings="nyc_trees.settings.test"` every time I run the unit tests though.  The alternative is to wrap this logic up in `scripts/manage.sh`