Fixed #47: use a database index when generating proxy binding in Garner::Mixins::Mongoid::Identity with multiple Garner.config.mongoid_identity_fields.
Merge pull request #49 from dblock/issue-47-limit-one

Fixed #47: use a database index when generating proxy binding in Garner::Mixins::Mongoid::Identity with multiple Garner.config.mongoid_identity_fields.