Rails 3.2 - Allow auto_increment columns to be part of composite key (MySQL only for now)

I saw that a couple months ago in pull request https://github.com/drnic/composite_primary_keys/pull/147, @krbullock made a commit to include this feature for ar_3.0.x, here is the change for 3.2. Additionally I made it so you can customize the auto incrementing column.