creating, updating and deleting tags without assigning them

src/python/PyBis/pybis/attribute.py

src/python/PyBis/pybis/definitions.py

src/python/PyBis/pybis/group.py

src/python/PyBis/pybis/pybis.py

src/python/PyBis/pybis/tag.py

src/python/PyBis/pybis/things.py

src/python/PyBis/pybis/utils.py

src/python/PyBis/tests/test_group.py

src/python/PyBis/tests/test_tag.py