Cookbook Privacy

:convenience_store: 

This makes Supermarket aware of a flag called `privacy` in a cookbook's metadata.  If present, and Supermarket is configured to enforce privacy, then the cookbook upload will fail.  This will require a change to the cookbook and also a change to chef itself, before it will work.  PRs for both of those will be forthcoming.  Note that I had to use `privacy` instead of `private` because `private` is a reserved word in Ruby and that wouldn't work in `metadata.rb`.  I felt consistency everywhere was important.

Closes https://github.com/opscode/supermarket/issues/832