Add docs about empty_values (close #211).
Fix abstract managers patching (close #212).
Fix proxy model handling.

Previously, proxy models would receive transaltion fields twice, as inspeceted by model._meta.get_fields_with_model()
Fix travis and tox files as Django 1.6 is released.
Updated changelog.
Added Jacek Tomaszewski to documentation copyright.
Prepared 0.7.2 release.