Added autocreation of state scopes for Mongoid, including RSpec tests

The interface mirrors the ActiveRecord support for the same, with the exception of the with_state_scope method (public for Mongoid) and calculate_in_state (not implemented for Mongoid)

Summary of changes
- Added the following methods to persistence/mongoid_persistence.rb
  - find_in_state
  - count_in_state
  - with_state_scope (protected in AR, would argue it should be made public there.
  - I did not port calculate_in_state, due to AR vs. Mongoid API differences; I made with_state_scope public instead. (I think it should be public for AR as well.)
- Moved AASM::Base patch which aliases scopes in persistence/active_record_persistence.rb to persistence/base.rb, and added a Mongoid detection clause to the if statement (in addition to AR). Also deleted commented code from the original method.
- Added 'mongoid' as dev dependency to gemspec
- Created two test files (mongoid_persistance_spec.rb and mongoid_models.rb) covering all my changes.