Configure Tox and Use It to Drive Travis

* Configures Tox to correctly run all of the items in our test matrix
* Updates ``.travis.yml``so it dispatches the testing logic to Tox
* Work around the bug where pytest-xdist doesn't detect if there was an error while collecting tests by making an explicit test collection test
* Test that our documentation builds correctly and without warnings

Essentially I got tired of remembering that I needed to run ``py.test`` instead of ``py.test -n 8`` on Python 3.2 and that I needed to run ``PYTHONHASHSEED=0 py.test -n 8`` on Python 3.3. This makes it so the command is always ``tox -e pyNN``. Additionally it consolidates the testing configuration in one place (tox.ini) so that we only need to maintain one configuration. Finally we make sure that our documentation always builds because we want good documentation.