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
Rename assocation/dissocation commands to org add/remove user.
Merge pull request #19 from opscode/ssd/rename-commands

Rename assocation/dissocation commands to org add/remove user.
Fix typo in README (user->organization)
updating README.md

Adding more instructions for running knife opc on the server itself, to improve clarity
Update readme to mention chef-server-ctl wrapper commands
Update installation directions
Add Chef Server 12 to the description
chef-server-ctl only exists on Chef 12
Add an `opc org edit` command.
Use early exit rather than if-else to improve readability.
Merge pull request #22 from opscode/ssd/add-org-edit

Add an `opc org edit` command.
Added command knife opc user password.

Command either updates the password for a user or, if --enable_external_auth is passed instead of a password, it will enable ldap for that user.
Merge pull request #20 from opscode/tc/add-password-cmd

Added command `knife opc user password`.
Add opc org user group add command.