Update setup.py to use pbr and start using tox with coverage
Split test_callbacks into two files
Rewrite test_method_callbacks to illustrate bug

This is the bug referred to by issue #1 where callbacks are registered
to the class instead of to the instance.
Remove confusing line from README
Add docstrings to package and decorator function
method callbacks are registered on instances

Fixes #1

Since the callback_registry is stored on the instance you can now have
multiple instances with different callbacks registered.
Update travis.yml and add coveralls support