obis: implemented addref and clone commands

src/python/OBis/obis/dm/command_result.py

src/python/OBis/obis/dm/commands/__init__.py

src/python/OBis/obis/dm/commands/addref.py

src/python/OBis/obis/dm/commands/clone.py

src/python/OBis/obis/dm/commands/openbis_command.py

src/python/OBis/obis/dm/commands/openbis_sync.py

src/python/OBis/obis/dm/data_mgmt.py

src/python/OBis/obis/dm/git.py

src/python/OBis/obis/dm/utils.py

src/python/OBis/obis/scripts/cli.py