Renamed models to remove the 'Railway_' prefix. Also updated points/tracks/signals
 to have a compound key of interlocking:site_id; this should allow us to reuse
 interlocking identifiers where the name makes sense, but still scopes it to
 a unique site.

Modifications have also been made to the SignalsController object to provide
 a list of all signals for a given interlocking:site combination, with the
 facility to add new signals also being provided. Next up is to add the #new
 action to the controller and provide the view, followed by the #create and #show
 actions along with the #show view.