Picky representer

Added the ability to select (with a :pick option) which fields/methods/attributes are to be represented when constructing a Representer instead of always having the same set. That way what's written in the class body can be treated as a definition of what's possible and you can select what you want at "runtime". It should also be easier to create actions where the client can select which fields he's interested in.

I hope you don't mind me taking liberties with the Gemfile and tests so that it all passes with no warnings on newest gem versions.