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.
Can you ignore this warning and allow the code to continue add a user to admin group?  Otherwise user has to dissociate first if forgot to add `-a` the first try.  Thanks.