Merge pull request #16 from gidoca/master

Use Django-internal six copy
Only use query_time when it has explicitly been set via as_of
Make adding several many-to-many related objects at once work correctly
Update docs to mention that it doesn't work with multi-table inheritance
Merge pull request #17 from brki/only-limit-with-querytime-if-specified

Only use query_time when it has explicitly been set via as_of
Merge pull request #18 from brki/fix-many-to-many-multiple-add

Fix many to many multiple add