Refactored to Master

I think I can merge this in now. No work is being done on the old master branch any more. 

This completely changes the entire thing. 

* The single file is split up into a number of new modules relating to different APIS:
 * `people.py` contains the People API, as well as a couple of bits from other APIs that relate to people.
 * `contacts.py` contains the People Contacts API
 * `lists.py` contains the Lists API.
 * `tags.py` contains the People Tags API. 

These all inherit basic functionality (response checking, authorisation etc.) from `nb_api.py` 

Also there is a `nationbuilder.py` module that should be used as an entry point for using nbpy.