Rename tests to better comply with test-discovery

as a step towards using py.test rename the unittests to format
discoverable by py.test
Change the tests to be run by py.test
Rewrite tests: use py.test syntax instead of nose
Add testing with py.test: python setup.py test

added the option to use python setup.py test according to
http://www.jeffknupp.com/blog/2013/08/16/open-sourcing-a-python-project-the-right-way/
Change travis to use pytest and coveralls