Log registrations from tor

This logs any attempt to register at www.theguardian.com via tor - as requested by infosec
(1) An agent maintains a set of possible tor exitnodes for profile.theguardian.com
(2) Implements a switch to control whether registrations should be logged.
(3) If the switch is turned on then any registration attempt will check whether their ip is a tor exit node and if so log their email address,

@gklopper - I've demo'd this to Dave B and Vishal.  
`final` is superfluous here as it's already an `object`.