introduce case insensitive authentication by email, closes #1261

Supported ORMS: activerecord, datamapper, minirecord, mongoid, mongomapper, sequel
ORMs not supporting case insensitive search: ohm
Cryptic ORMS: couchrest
Merge pull request #1344 from padrino/mixed-emails

introduce case insensitive authentication by email, closes #1261
skip padrino-cache with mongo on rbx engine