Change how required settings are handled

Certain settings are going to be required for the application to run.
Currently `SECRET_KEY` is the only one, but the list will most likely
grow over time. The application factory allows for the settings in
`settings.py` to be overridden a couple of different ways. Required
settings should be checked after all of the overrides have been
processed.

This is being implemented in the form of a helper function that checks
for the required keys. Because `settings.py` will add the key to the
configuration, `DOES_NOT_EXIST` is a value that can be assigned to a key
that is intended to be required. `check_required_settings` will respect
both the absence of the key and the value `DOES_NOT_EXIST`.

Along with this change comes some tests. These two tests are the
groundwork for the eventual suite of tests. `tox` can be used to run the
tests. `tox` requires a `setup.py` in order to install the package into
the virtualenv, so a minimal one is being added for that purpose.

Fixes #32