contractual

This gem provides limited support for the utilization of interfaces in Ruby. The approach here is nearly idetnical to one suggested by Mark Bates at http://metabates.com/2011/02/07/building-interfaces-and-abstract-classes-in-ruby/. It didn't seem like this had been turned into a gem yet, so I thought I might go ahead and put it together in case others found the technique as helpful as I had.