Add a tox configuration file

Tox is a useful tool to run automated cross-platform tests for different Python
versions to ensure that the module runs on all of them, see
http://tox.readthedocs.org/en/latest/

For the following Python versions automated testing using tox was enabled:

- Python 2.6, 2.7
- Python 3.2, 3.3
- pypy
Merge pull request #45 from aepsil0n/feature/tox

Add a tox configuration file