Add tests to detect the error.
Assign the correct tree id to root nodes.
Make the package installable at the same time of its requirements.

`python setup.py --version` now works even if sqlalchemy is not
installed.
Python 3 compatibility.
Merge pull request #26 from GaretJax/22-setup

Make the package installable at the same time of its requirements.
Merge pull request #25 from GaretJax/23-tree_id

Set the correct tree ID for root nodes
Merge branch 'master' into develop

Conflicts:
	sqlalchemy_mptt/tests/tests_events.py