Ticket #16920 -- Added GenericRelation tests

First test ensures related_name may not be set with GenericRelation.
Second test ensures that two models with the same name, but living in a
different app, do not cause a related_name clash when both defining a
GenericRelation to the same related model.

Test Cleanup

Refactored for django 1.7 app-loading mechanism

Ensure GenericRelation fields are check()'ed

More tests added

Refactored _check_accessor_clashes and _check_reverse_query_clashes back
to _check_clashes with a check_related_query_name param

Clean up according to review

Changed test from assertRaises to assertRaisesMessage for more clarity