Add option to add contacts as member to groups

Currently "samba-tool group addmemers" can just add users, groups and computers as members to groups. But also contacts can be members of groups. This patch set adds the functionality to add contacts to groups. Since contacts have no sAMAccountName, it's possible that there are more than one contact with the same name in different organizational units. Therefore it's necessary to have an option to handle group members by their DN.

To get the DN of an object there is now the --full-dn option available for all necessary commands.

The MS Windows UI allows to search for specific types of group members when searching for new members for a group. This feature is included here with the new samba-tool group addmembers --object-type=OBJECTYPE option. The different types are selected accordingly to the Windows UI. The default samba-toole behaviour shouldn't be changed.