Fixes for mongo_mapper adapter. Add specs.

Cherry-picked from juggler/cancan/commit/0a2556bf557334a8b2e002ad573692d47dc9d54a

Conflicts:
	Gemfile
	lib/cancan/model_adapters/mongo_mapper_adapter.rb
	spec/cancan/model_adapters/mongo_mapper_adapter_spec.rb
MongoMapper ~> 0.12.0
Use WithModel only if it's defined (MODEL_ADAPTER being ActiveRecord)
MongoMapperAdapter now supporting scopes