Policyfile CLI

Implements `chef install` and `chef push`. Before we merge/ship, should probably document how to configure chef-client for policyfile mode. Also need to ship a new chef-client alpha gem with the WorkstationConfigLoader in it before we can merge and build correctly.

/cc @opscode/client-engineers 
Good call. I like to use memoization to make computation more declarative, but there isn't any significant computation happening here.