Python 3 port

Hi!

I created a version that actually runs under both Python 2 and Python 3, as far as I can tell.

I replaced __oauth2__ and __httplib2__ with __requests__ and __requests-oauthlib__ which both are compatible to Python 2 and 3. The testsuite completed successfully when I ran it under Python 2.7 and Python 3.3. Some tests were added by me, as well.  

I'm using the package in a Trello integration for our website that makes use of OAuth (user grants privileges to our app and we work with the user's boards) and so far it seems to work. But naturally it'd be nice if someone else could verify this.