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

Docs are coming.  If this could be merged ASAP, the good docs would be an example in #59
where does `in_func` come from?