Add support to run tests with py.test and coverage. #256

To be able to use this configuration one need to run:
```
# tox -c tox_pytest.ini
```
This configuration file setup the pytest-cov plugin and it is an additional dependency. 

It is possible to tests with some versions of python, to do this the command is:
```
# tox -e py27,py34,pypy -c tox_pytest.ini
```

Will run py.test with the python2.7, python3.4 nd pypy interpreters, for example.
@pfmoore do you think changing the default is out of scope for this PR?