Test and fix for second grandchild bug.

Bug is, second grandchild of a given child will get cast to the child
rather then the grandchild.
Fix tox.ini so runtests.sh works.
Merge pull request #50 from CrazyCasta/tox-fix

Fix tox.ini so runtests.sh works.
Merge branch 'master' of git://github.com/carljm/django-model-utils into grandchild-fix
Update AUTHORS and changelog.
PassThroughManager calls superclass `get_query_set`.

As discussed in #31, PassThroughManager and
PassThroughManager.for_queryset_class() would ignore the superclass
version of `get_query_set`.
Tweak AUTHORS and changelog.
Remove misinformation from changelog; oops.
Merge branch 'master' into fk-fix

* master:
  Remove misinformation from changelog; oops.
  Tweak AUTHORS and changelog.
  PassThroughManager calls superclass `get_query_set`.
  Update AUTHORS and changelog.
  Fix tox.ini so runtests.sh works.
  Test and fix for second grandchild bug.
Doc updates for FieldTracker -> ModelTracker transition.