Cleanup warnings and update documentation.
Handle None values returned by Form's clean(). They are valid starting from Django 1.7.
Merge pull request #217 from spectras/fix/clean-217

Allow clean() method of TranslatableModelForm's subclasses to return None. This is allowed by Django starting from v1.7.
Pick MySQL compatibility tests and fixes from 0.5.2. See #212.
Rework kwarg and query translation so they accept deep query strings