Added some basic testing for TranslationManager.any_language().
Made get_translation_aware_manager create an TranslationAware manager
from the model's default manager. It works by copying the class and
replacing any occurence of Manager or TranslationManager in the bases
with a TranslationAwareManager.
OPEN ISSUE: That will break if a manager class explicitly includes both.
That should not happen, but maybe it would be worth checking?
Added more useful message when default manager inherits both Manager and TranslationManager
(they both get replaced by TranslationAwareManager, which raises a TypeError).

In the future, we could replace only the closest TranslationManager ancestor with TranslationAwareManager
but this requires much more complex code to handle something that is not supposed to happen in the first place.
Added tests to check availability of custom manager functions, after
a language() calls, after a any_language() call, and on a translation aware manager.
Added test for custom queryset methods on querysets returned by
translation aware managers.