Make ActiveAdmin ORM agnostic (i.e. works without loading ActiveRecord)

1) Add ORM-specific loaders:
- lib\active_admin\orm\active_record
- lib\active_admin\orm\mongoid

2) Move Comments plugin to ActiveRecord ORM directory. Comments support is AR-only for now.

3) Add ActiveRecord/Mongoid type checks to DisplayHelper#pretty_format(object) method