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.
We could encourage UI test writing cleanliness from the get go by creating a base class with some helpers:

```python
class NycTreesSeleniumTestCase(SeleniumTestCase):

    def wait_for_textbox_then_type(self, selector, text):
        self.wait_for_element(selector).send_keys(text)

    def type_in_textbox(self, selector, text):
        self.sel.find_element_by_name(selector).send_keys(text)

```