Improvements for Rails and ActiveRecord Integration

1) Add validation tests (+ error class in order to pass)
2) Fixed login ticket lifetime bug (maximum_unused_login_ticket_lifetime was not used in login ticket validation)
3) Add remember_me functionality in the core
4) Change how service tickets are generated. Now each service tickets inherits from TicketGrantingTicket
in a way to imitate activerecord's belongs_to/has_many.
5) Other minor improvements
6) Tested with rubycas-server-rails engine gem and rubycas-server-activerecord gem