Mongoid 4.0 support

issue #120

Hi, I've added support for mongoid, but without associations. 

Here what I've done:
- active_record, arel codes moved into `adapters/active_record`
  * ransack classes that hardly depend on AR/Arel are moved inside `adapters/active_record/ransack`
- mongoid codes are inside `adapters/mongoid`
- created `Attribute` class for mongoid that imitates `Arel::Attributes::Attribute`
- all existing ransack specs remained, added specs for mongoid
- disabled joins for mongoid. there are no specs for mongoid/joins
- added rake tasks
  - `$ rake` runs active_record specs. **default**
  - `$ DB=mongodb rake` runs mongoid specs