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 think the reason was the user command I had used didn't set the privileges for all databases. Should have instead been::

    mysql> CREATE USER 'your_user_name'@'localhost';
    mysql> GRANT ALL PRIVILEGES ON *.* TO 'your_user_name'@'localhost';
    mysql> CREATE DATABASE dtc;