Allow overriding Registration step redirect targets.

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.
There's also a Registration companion object, so that the controller
can also be routed as before.