Feature Request: Make it ORM-agnostic

Created by: mrbrdo

We are using Sequel and are looking for a decent admin (which is a problem even if using ActiveRecord). Since this project still seems young and showing promise, it would be nice if you could extract ORM-specific operations into a separate module so they can more easily be changed for a different ORM.

I think the only difficult thing I found in respect to Sequel was assoc_ids getter/setter which does not exist in Sequel. But it's not that big of a deal as it is not too hard to implement (although it would be nicer if this could be handled in the controller instead of unconditionally offloading it to the model, which may not be able to handle it in some ORMs).

I got most of it working with these monkey-patches: https://gist.github.com/mrbrdo/56da0dd984b4191a7273

I would be willing to provide Sequel implementations for such a module.

Cheers