added collection adapter for active record

Referring to https://github.com/ianwhite/orm_adapter/issues/27

This is just a discussion starting point. As you may have seen, I add the collection adapter only to ActiveRecord. This is because I wanted to discuss with you the gem extension, the validity of the additional API, and if we agree on it, some corrections to the specs/internal implementation I added, and other possible extensions to the general purpose API, before writing adapters to other ORMs.

So, this is my point of view: I think that the collections not only represent an abstraction present in all ORM implementations, they also offer some general CRUD scoped methods that can be useful to the further extension of certain gems. From that perspective it would be interesting to bring them all together. 

So, please don't close the request before we discuss this further. I think it would be more interesting to add this to the existing gem instead of adding a new "collection_orm_adapter" gem.