Feature/fallback refactor

__Work in progress__
This pull request tracks the work in branch feature/fallback_refactor.

The goals of those changes are as follows:
* Cleanup the manager zoo. There are too many, and they serve no purpose. __[done]__
* Allow full customization of queryset classes used for `language()`, `untranslated()` and `get_queryset()`. __[done]__
* Implement missing features of the `FallbackQueryset`. I have `get()` in mind but it is likely there are others. __[todo]__
* Have fallbacks that to not eat the database alive. __[proof of concept works]__
* Make hvad compatible with the new `QuerySet.as_manager()` method introduced in Django 1.7. __[todo]__

This should be done with minimal impact to the API. Some changes to the internals are unavoidable though, thus the separate branch. Help is welcome, comments as well.