Add Chef::Org class

This moves common code in the org management commands to
to a a Chef::Org model library.

Additionally, it adds a --admin flag to the association command
to make it easy to associate a user and add them to the admins group.
@marcparadise Awesome, thanks for the info!  I'm going to keep this method for now as it works on both, but if this ever gets into core knife, it would be nice to use the new API.