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.
Add simplecov for coverage stats
Use UI#msg rather than the non-existant UI#puts
Don't exit immediately on association failure

If a user forgets to use the --admin flag, not exiting immediately on
409 will allow them to simply run the command again.
Merge pull request #17 from opscode/ssd/chef-org-class

Add Chef::Org class