Added a tox ini file that uses py.test

This is basically a convenience PR. It adds an extra ```tox``` configuration file that uses ```py.test``` to run the tests. The benefit is that the ```py.test``` output can be easier to read when looking for failure information (which has been a problem while trying to diagnose the Windows failures).

I actually much prefer ```py.test``` for testing, and I think it would be useful to use it for the actual tests - but that's not what this PR does, the existing tests are unchanged here.

To use this file, just do ```tox -c tox_pytest.ini```.