Add Chef::Org model class

Chef Server 12 added multi-tenancy to the open source Chef Server. An
organization is the Chef Servers name for a tenant.  The Chef::Org
class provides a model of an organization to facilitate writting knife
commands that can interact with (create, destroy, list) Chef
organizations.

This implementation is copied directly from knife-opc, the knife
plugin currently in use by chef-server-ctl:

  https://github.com/opscode/knife-opc
This method came along for the ride because it was useful for the `knife-opc` project where this code was copied from.  It doesn't really make sense here in my opinion.  We should probably have a Chef::Group class that has an #add_user and #remove_user method.  I'll pull this out in my next commit unless someone things it makes sense to have group operations in this class.