Support for using aliased field names in combination with validates_uniqueness. (Non-aliased names continue to function as before.)

Specs cover:
- aliased field names in combination with validates_uniqueness
- the presence of aliased field key in the errors hash
- aliased field in embedded documents
Merge branch 'bug-alias-unique' of https://github.com/johnnyshields/mongoid into 3.1.0-stable