Open source python client

Here's tumblr`s internal python client.  We use it in production, so what's here is stable, though there are a bunch of API endpoints that aren't covered yet.  We'll be iterating, and accepting any improvements that external contributors choose to give us :wink:
Once this has approval, I'll be also publishing it on [pypi](https://pypi.python.org/pypi), for easy distribution and installation.