Mongoid 4.0 support

issue #120

Hi, I've added support for mongoid, but without assosiations. 

Here what I've done:
- moved active_record, arel codes into adapters/active_record
- created adapters/mongoid
- created Attribute class that imitates Arel's Attribute
- migrated ransack's specs for mongoid
- disabled joins for mongoid. there are no specs for joins
- added rake tasks
  - `$ rake` run active_record ransack specs
  - `$ rake mongoid` run mongoid ransack specs