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 
It looks like you're lazy initializing the ui here.  Is there a reason for that, rather than doing it in the `initialize` method?  If so, will you add a comment (unless this is some pattern I'm just not familiar with)?