add support for user groups
each host can be associated with a user or a group, allowing to send email notifications to the right user or group of users.
Fixes #170 - implement usergroups
Added MVC files
Added the routes as well
Added the program logic and additional views
Added RESTful actions for update and create
Added validations to ensure that usergroup.name and user.login do not collide
Added "recipients" method to users and usergroups. This returns a list of addresses.
Usergroup#recipients will follow all nested usergroups avoiding any loops and
return only unique addresses
Fixes #73 - Add usergroup support to hosts
Fixes #244 - Remove ActiveScaffold from the usergroup pages
and finally - added owners to host edit page and other whitespaces and
mods by ohad