Generate service client stubs with a pluggable protocol architecture

This pull request adds support to the Beefcake generator for creating client stubs, as well as a subsystem for providing pluggable backends such that users can send/receive messages with services over whichever protocol they want.

I've implemented an example protocol here: https://github.com/bradhe/beefcake_go_rpcgen

I'm open to discussion on any point here--I'm really impressed by the simplicity and layout of Beefcake. If anyone has suggestions as to how this can be improved, I'm all ears!

For better or for worse, this includes PR 23 as I needed it in my code, too!