Refactor of backend logic & settings modules

Now that the competition is over I can show some of the stuff I've been working on in my branch.

There are two things here I was working on:

**1. Refactor of the settings module(s).**

There is now a ``smartclip/settings`` directory instead of a ``smartclip/settings.py``. Inside ``smartclip/settings`` you'll find ``base.py``, ``prod.py``, and ``vagrant.py``. Modules are just more easily handled this way instead of needing custom settings files (e.g. ``local_settings.py``) just for local dev. You still have them, but they're minimal.

This is intended for use with the [smartclip-dev](https://github.com/mattdeboard/smartclip-dev) repo for a local dev environment.

**2. Refactor of backend logic.**

All logic around instantiating the api client has been moved out into a separate ``smartclip.backends`` module. Right now there's only the backend for SmartFile, but the objective is to make adding backends for other storage solutions simple.