Resolve database primary key and LDAP identifier conflicts.

This pull request add two type of identifiers. One for your database model and one for your LDAP model.

Usually the LDAP identifier is the `username` attribute, string/varchar datatype.
And the Database identifier is the primary key of your table (`id` column by default), integer datatype.

This feature is really useful when you need to add some relations to your User model (E.g. attaching roles).