Cleanup for better Mongoid support

- Mongoid now requires to include Mongoid::Phony module, rather than auto-including it into Mongoid::Document. (This is the convention for nearly all other Mongoid plug-ins)
- Refactor phony_rails_spec.rb to test both ActiveRecord and Mongoid
- Refactor lib/phony_rails.rb so the core PhonyRails::Extension module uses ActiveSupport::Concern. Correspondingly, AR::Base now includes rather than extends the module.
- Change .gemspec dependencies so activesupport is a prod dep, and activerecord/mongoid are development deps
- Update README.md for Mongoid