support colander schema binding with request object
this enables the use of deferred validators with cornice 
and colander schemas
update contributors
Python3.3 compatibility.

- USERS.keys() => list(USERS.keys())
- Replace `isinstance(x, basestring)` idiom with something more Python 3
  compatible
- Changes to accomodate that request and response bodies are byte strings
  with Python 3.
- Add py33 to tox.ini and travis.yml
- Add Python 3 Trove classifier