Use `Mongoid::Matchers#matches?` instead of  a database query in `MongoidAdapter#matches_conditions_hash?`

allow model adapter to override condition hash matching in Rule, also clean up Mongoid adapter and specs

Automatically add `accessible_by` to Mongoid Documents to match CanCan behavior for ActiveRecord and DataMapper.

Previously, CanCan::ModelAdditions had to be included in each and every Mongoid document separately. Also removed manual include of CanCan::ModelAdditions from Mongoid documents in Mongoid adapter specs.

removing Mongoid::Components hack, tests are passing without it, add tests if this is actually needed

moving model adapter specs into their own directory with MODEL_ADAPTER environment variable for choosing which one to run

This broke some of the mongoid tests and I don't know how to fix them.  Both packages
  define Symbol#in, and when you load them both things don't behave properly.  Hopefully
  someone more versed in mongoid can rewrite the spec to not depend on the Symbol extensions.

Fix bug with MongoidAdditions throwing a NameError when Mongoid is not defined by always checking if Mongoid is defined before referencing Mongoid-related constants

Also add spec for this bug