DRY up AR adapter code using a WhereBuilder

Hi Arndt
I finally managed to create a pull request. I just created a branch in my sferik/rails_admin fork and pulled in your master.
What do you think about the WhereBuilder? I somehow think its leading in the right direction, but can further be improved, e.g. by doing the same in the mongoid adapter and maybe pull up some common methods to an "abstract" class ...