prepare development / test environment for py 3.3

- add a tox configuration, using a requirements.txt that pulls py 3.3
  compatible flask/werkzeug code from git repos
- add py 3.3 to travis configuration
- add a note about the port to CHANGES
- add myself to CONTRIBUTORS
- add more specific python version classifiers to setup.py
tox: fix py33 test env, need py 3.3 compatible unittest2 there
port to python 3.3

all unit tests succeed on 2.6/2.7/3.3.

but more practical testing / more fixing is for sure needed.
if someone tries to fix something, just have a look at what's already
there in flask._compat (or werkzeug._compat) before reinventing it.
have a private _compat module to not depend on specific flask version
Merge pull request #67 from ThomasWaldmann/python3

Python3 port