Switch to pytest

This switches from using `python setup.py test` to run tests to using [pytest](http://pytest.org/latest/) (and [pytest-sugar](https://pypi.python.org/pypi/pytest-sugar)).

Advantages are that pytest gives you more control over running tests like only running selected tests and gives you options of nicer output, easily displaying coverage or generating JUnit and Cobertura XML files that can be consumed by Jenkins, etc.. [pytest-sugar](https://pypi.python.org/pypi/pytest-sugar) in particular gives you nice output with progress info and shows failures immediately instead of waiting until the end.

Give it a shot and see if you like it.

If not, no biggie.

```
$ tox
...
  py26: commands succeeded
  py27: commands succeeded
  pypy: commands succeeded
  congratulations :)
```

![screen shot 2014-04-10 at 10 57 34 am](https://cloud.githubusercontent.com/assets/305268/2671370/ae024026-c0d9-11e3-9a4f-7eb7f4b3f676.png)