Merge pull request #14 from swisscom/reduce_warnings_w_django17

Reduce warnings w django17
Use Django-internal six copy
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
Allow direct assignment of values to a many-to-many relationship field
Update docs to mention that it doesn't work with multi-table inheritance
Make special database columns uneditable
Merge branch 'master' of https://github.com/swisscom/cleanerversion into uneditable
Django 1.6 compatibility fix for m2m direct assignment
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
Removed the unused 'set_as_of' method; Removed the extra argument to 'add_as_of_filter'
Added further tests for adaptions in PR #18
Merge pull request #18 from brki/fix-many-to-many-multiple-add

Fix many to many multiple add
Merge remote-tracking branch 'origin/master' into cleanup_querytime_limit

Conflicts:
	versions/models.py
	versions/tests.py
Merge pull request #20 from brki/allow-direct-assignment-of-m2m-relationships

Allow direct assignment of m2m relationships
Merge remote-tracking branch 'origin/master' into cleanup_querytime_limit

Conflicts:
	versions/tests.py
Made setting M2M relationships slightly more tolerant and unified the way M2M properties are accessed by the new code parts (via the __get__ method)
Removed six; it shouldn't be required anymore since PR #16
Fixed compatibility issue with Django 1.6 and ValuesListQuerySets