Bulk update or create

[jonathan-s]

This should fix #49.

There is probably still improvements to be made to this PR but the initial approach seems to work for me. So I'm submitting this for review.

This PR basically adds the functionality of using update_or_create to the BulkUpdateManager. Which has been requested for some time.

The approach I took to accomplish this is to first separate all objects that have an id in a batch. Then make a select statement to check that they actually exist in the database. I update all of these objects using the already existing bulk_update method.

The leftover objects didn't exist in the database, thus they need to be inserted. Since technically you can create an object and still get a collision the insertion of those objects take place with a custom crafted upsert.

So for instance if you have a field that is unique it will avoid a collision with that uniqueness and instead do an update instead of an insertion.

[jonathan-s]

Making a note that travis is actually only testing postgres at the moment because of this change

[jonathan-s]

@aykut and @arnau126 Might want to review this

[tony]

This may be worth looking at: http://django-postgres-extra.readthedocs.io/manager/ (Postgres only)