Rollback password file on createUser fail. Updated createUser schema.

Rollback password file change when createUser fails (ie. user already on the db).

Updated createUser schema to MongoDB >= 2.6, schema version: 3. http://docs.mongodb.org/manual/release-notes/2.6-upgrade/#authentication
http://docs.mongodb.org/manual/release-notes/2.6-upgrade-authorization/#upgrade-authorization-model

Fixes issue #39 after MongoDB 2.6.x update and upgrade.