Update to tox 1.8 and update documentation

Updated ``tox.ini`` file to make use of environment generators introduced in tox 1.8.

Additionally added documentation into ``CONTRIBUTING.rst`` to explain how to install and configure the local databases needed to run the tests.
I can't reproduce this need to have a pre-created ``test_dtc`` database on MySQL, for any version of Django.

The settings files give the database name ``dtc`` (and Django requires that this database actually exist), but Django automatically creates a test database named ``test_dtc`` and uses that for the tests, and then destroys it after running the tests. But you shouldn't ever need to create it yourself. I'm able to run the full tox suite successfully, and my MySQL server only has a ``dtc`` database, not ``test_dtc``.

This system does require that your database user have permission to create databases - it may be that this is what you are lacking.