Alternative approach to fix ticket 56.
Based on discussion #49
This approach doesn't change AutoField behavior. Only those who need extended int range may use it.
It keeps values ranges consistent between databases. Also it works nice with related fields.
To me it seems this needs some tests, mainly one for checking that the foreign key to a bigautofield is created properly as bigint. As per the policy of "commit or close when reviewing" I am closing this pull request.