Allow disabling the primary index on objects in order to prevent extraneous index maintenance for mid level cached associations
Explicitly assert that with the primary index disabled, deletes aren't issued to it, and make the primary index disabling macro complain if a secondary index has been created.
Merge pull request #53 from Shopify/disable_primary_index

Allow disabling the primary index on objects in order to prevent extraneous index maintenance for mid level cached associations