Test Python 2.6, 2.7, 3.3 and 3.4 with Travis

If we use tox to run the tests we can verify against multple Python
versions with Travis. This should make it easier to identify issues
with version compatability.
Merge branch 'travis-multi-python' of https://github.com/d0ugal/simmetrica into d0ugal-travis-multi-python

Conflicts:
	requirements-dev.txt