make model class configurable

this adds an install generator (`rails g instant_login:install`) that creates an initializer and a migration file(later is configurable) 
we make the user class configurable in the initializer and only extend the specified class with the required methods for the specific DB abstaction (currently only AR) 
Wäre hier ein `model_class_name.constantize.table_name` nicht sinnvoller? Falls jemand `table_name` aus welchem Grund auch immer anders gesetzt hat... :)