Fixes for test_update_command

Note: this is a follow up to #276, the first 5 commits are exactly the same. The fixes here do depend on the enhancement proposed there, but in case you don't like the command app-selection feature, with some more work it should be possible to work around it.

I've been seeing various failures of the ``test_update_command`` for some time already, however none of them can be replicated with an empty project. They turned out not to be related to the command itself; rather the test acts as a beacon of side-effects and other quirks as its processing goes over all registered models. The failures fall into 3 categories:

* losing references to classes in ``modeltranslation.tests.models`` other than models, due to the hard-core reloading during the test base setup; usually apparent as some unexpected ``None`` during update of one of the custom manager models, for example ([M + MT](https://github.com/wrwrwr/mezzanine/compare/feature/modeltranslation-integration-1.7) + [1.7](https://github.com/django/django/commits/stable/1.7.x)):
```
File "/modeltranslation/tests/models.py", line 266, in get_queryset
    sup = super(CustomManager, self)
TypeError: must be type, not None

File "/modeltranslation/tests/models.py", line 294, in get_queryset
    return CustomQuerySet(self.model, using=self._db)
TypeError: 'NoneType' object is not callable
```
* Update not being effective, due to the command module being loaded before modeltranslation tests setup and sticking to some project's default language ([M + MT](https://github.com/wrwrwr/mezzanine/compare/feature/modeltranslation-integration-1.7) + [1.6](https://github.com/django/django/commits/stable/1.6.x)):
```
File "/modeltranslation/tests/tests.py", line 1903, in test_update_command
    self.assertEqual('initial', obj1.title_de)
AssertionError: 'initial' != u''
```
* Update trying to update user-models registered with project's languages different than the tests set (e.g. same as the last one plus this branch without 1aeaf69):
```
File "/django/db/models/sql/query.py", line 1283, in names_to_path
    "Choices are: %s" % (name, ", ".join(available)))
FieldError: Cannot resolve keyword 'something_de' into field. 
Choices are: <correct translation fields for project's language set>
```
In my experience, while working with the test code it's easy to run into other related issues, most perfidious are those caused by different parts of the system using different copies of ``translator``. One hilarious was from an attempt to replace models reloading with lazy registration -- a ``NotRegistered`` error during ``register``.

The idea is to avoid some module reloading in test preparation. The heavy changes are in the first two commits. First, in 9a74829, ``reload_app`` is asked to clean the translator registry from models that got reloaded, secondly 6c673cd refactors the test base to avoid reloading the translator module.

There is still some reloading -- of ``django.utils.translation`` and ``modeltranslation.settings``. The first one didn't happen to cause any trouble so far and as long as nothing imports or stores references to objects from modeltranslation settings the implemented module-level reference fixing should be general enough.

The only actual (at least as intended) test changes are in the last two commits: 383fa4b and 1aeaf69.

A nice thing is that with this branch you can run the Mezzanine tests together with modeltranslation suite without any failure, on Django 1.4 - 1.7, with or without South.