Add user group and server group

Implemented the following list of functions but have not tested yet
* Ability to create new ServerGroup
* Ability to add/remove servers to ServerGroup (with PUT)
* Ability to delete ServerGroup
* Ability to list all ServerGroups

* Ability to create new UserGroup
* Ability to add/remove Users to UserGroup (with PUT)
* Ability to delete UserGroup
* Ability to list all UserGroups

Especially the one that add/removes servers/users to Group, needs review and help 

Or should it be able to add a few users/servers one time instead of 1 by 1?