Update incorrect examples
Make ORM loading on demand, remove forced ActiveRecord ORM loading when no config specified.

This also should fix issues when p_a is used in Rails apps with multiple ORMs. #89
Merge branch 'master' into rails4
Relax dependencies to include Rails 4.0.0rc.

This change also includes Mongoid from `master` branch, because Mongoid 3.1.X requires different ActiveModel than ActiveRecord.
- self.abstract_class no longer needed in Rails 4
- removed unnecessary touch to config since master now loads ORM on demand