New Hostgroup.delete() incl. cleanup_related_items

Hostgroup.delete() removes references to a deleted hostgroup in
- host.hostgroups
- hostgroup.hostgroup_members
- service/host_escalations (both hostgroup_name and dependent_hostgroup_name)
delete() leaves services that are still required

hostgroup.delete() and host.delete() only remove services if
recursive==True and service is not mapped to any other object.
Merge pull request #46 from gradecke/master

new Hostgroup.delete() incl. cleanup_related_items