Add coverage and refactor test execution

- Use nosetests task rather than test as test support is deprecated,
  see;
  https://code.google.com/p/python-nose/issues/detail?id=219&can=1&q=with-doctest%20python%20setup.py%20test&colspec=ID%20Type%20Status%20Priority%20Stars%20Milestone%20Owner%20Summary
- Move nosetests flags into setup.cfg as per the docs, see;
  http://nose.readthedocs.org/en/latest/api/commands.html#nosetests-setuptools-command
- Switch on coverage and xunit output
Add coveralls support to travis build
Merge pull request #16 from alphagov/add-coveralls

Add coveralls