add tox configuration, bumped version in setup.py, added classifiers
port to python 3.3 (and also support 2.6 / 2.7)

added 3.3 to tox, py3 classifiers to setup.py
added some compatibility wrappers
adapted doctests so they work the same way on py2 and py3
changed __repr__ so it gives the same on py2 and py3 (no "u" on py2)