py3k compatibility

Hi there

I've added initial Python 3 compatibility.
This is of course just for the client library so I can generate safe Thumbor urls in py3k projects as well. The Thumbor server itself still has to run in its own Python 2 environment.

Please review but do not merge this yet, as I'm still unhappy with this quality myself. ;)
No, this is perfectly fine (and way more readable anyway). It's also the recommended way to do in [modern Python](https://docs.python.org/2/tutorial/errors.html#handling-exceptions).