Using a specific cache from settings

Waffle will look for the setting WAFFLE_CACHE_NAME and if it exists,
use that cache instead of the default django cache.

This is done in `compat.py`, as Django 1.7 changes the cache getter API a little.

If you think this is a good approach I can add some docs and maybe some tests, except not quite sure how to test this yet.

resolves #122