Change to collect all factory options into a Meta class

Match how Django does it (looks cleaner to me, and it's in the ideas doc), putting all `FACTORY_*` options into `class Meta`. After the class is defined, the options are put into a `_meta` attribute on the factory.

Completely ignores compatibility with `FACTORY_*` style options. This is deliberate (and lazy on my part) to prevent clashes between new and old style. Deprecating the old style may have been better, but that's what major version bumps are for :)

Changed some of the argument names as I went. See `DefaultOptions` class in `base.py`.

Naturally, most of this change is just bookkeeping the arguments.