ENH : added testing helper classes

 - added KnownFail exception + plugin
   - copied from mpl/numpy
 - added fail_if decorator
 - added script `run_tests.py` to repo manage running the tests
 - updated .travis.yml to use new script
DOC : fixed typos in known_fail_if doc

 - fixed typos in docstring
 - added a few more comments
 - removed misleading comments
Merge pull request #81 from tacaswell/improve_testing

Will patch this into the current PR that @sameera2004 has later