Python 3 compatibility

Fixes for the Python 3 problems, which are:

implicit relative imports

unicode literals (Python 3.2).

I also add a tox.ini. Tox is in the easiest way I know to run tests in multiple environments - you just "pip install tox" then do "tox", and it builds all the virtualenvs as necessary.

I haven't been able to get a Python 2.5 binary installed on my machine yet, so that is not tested, but all the tests pass on Python 2.6, 2.7, 3.2, 3.3.

Thanks!