self.extend_orms should include ActiveRecord and Mongoid ORMs by default

- self.extend_orms is now [:active_record, :mongoid] by default
- ORMs will be skipped if the required gem hasn't been loaded, even if self.extend_orms includes that ORM
- Note that even prior to this commit the README.rdoc states that AR is included by default, but it is not currently
- I've updated the README.rdoc to clarify this behavior