Install cookbooks from Policyfile.lock.json

Adds library code that can install cookbooks directly from the lockfile. There is no end-user interface for this behavior (that will come once some other prerequisite functionality has been implemented), but the scripts in the test-scripts directory exercise the code in a real-world-ish scenario.
The policyfile DSL is this class: https://github.com/opscode/chef-dk/blob/master/lib/chef-dk/policyfile/dsl.rb and the methods a user is supposed to use in their policyfile are just the public methods on that class.