Easily run tests in a virtualenv with tox and nose

tox is a nice wrapper around virtualenv. It setup a new env that makes
sure you have a clean install without any module beside the standard
one. If you need more modules, simply add them to 'deps =' and tox will
install them from pypi.

nose makes testing easier, it find any files that looks like a test and
run them for you.

The envs are created in /.tox so ignore that directory entirely.