Install and document coverage and coveralls (fixes #9)

Fixes #9
I know you prefer py.test and I like it too. But I wonder if we should set the standard by using unittest instead. After all, assert works horribly if you *don't* use py.test and since nose is the default, it would be better to use `self.assertEqual(1 + 1, 2)` or `eq_(1 + 1, 2)`.