simple implementation of "implicit" sets

This is not really ready to pull but is here for comment.  It's a first stab at making the set API look more like the design on http://wagn.org/Packs+api, where the set (in views, events, and models) is implicit from the filename. 

rspec ./spec/lib/wagn/renderer_spec.rb:512
rspec ./spec/lib/wagn/renderer_spec.rb:516