Supporting python 3

I basically used [six](http://pythonhosted.org//six/) (and some other minor modifications) to fix compatibility issues. Since [PySO8601](http://pythonhosted.org//PySO8601/) didn't even installed with pip on a python3 virtualenv in my machine, I dropped it in favor of [aniso8601](http://aniso8601.readthedocs.org/en/v0.83/), that does basically the same thing, and supports python 3. Also added tox configuration to test in multiple versions.

PS: I couldn't help but notice that your last commit is more than a year old, so if you don't have interest in maintaining this project anymore, I do =)