Allow to override Registration.handle* redirect targets, for this change Registration to managed controller (since play 2.1)

Registration actions like handleStartSignUp etc. redirected to login, which is not always desirable. To allow users to change this the Registration object is changed to a class so that it can be used as a managed controller (which was introduced with play 2.1) and subclassed by securesocial users.

This change is based on the 2.1 branch.