Initial update_together attempt.

This won't work since multiple __metaclass__s makes Python sad. I don't
like the idea of this functioning as an extension of ModelBase as
opposed to a mixin, however.
Change UpdateTogether mixin to UpdateTogetherModel

This'll work, but it sucks for a number of reasons:
1) I'm replacing models.Model just to add one damn field to Meta
2) If anyone else needed to do the same thing (and I certainly have
places where I do this already in other projects) I've just screwed
them. You either need to cook up your own metaclass that inherits from
all of our metaclasses and then do the same for your models.Model or
only use one of our packages.

This is dumb.
Support Py3's new metaclass syntax.
Add whitespace.
Merge from develop.
Update documentation.
Documentation fixes.
Add `update_together` test.
Merge branch 'feature/update-together' into develop

* feature/update-together:
  Add `update_together` test.
  Documentation fixes.
  Update documentation.
  Add whitespace.
  Support Py3's new metaclass syntax.
  Change UpdateTogether mixin to UpdateTogetherModel
  Initial update_together attempt.