test expire session
Merge branch 'develop' into path-33

Conflicts:
	sqlalchemy_mptt/events.py
issue #33
Add level to expire session test
Update .travis.yml
Update README.rst
Update contribute.rst
Merge branch 'develop' of github.com:ITCase/sqlalchemy_mptt into path-33
Merge branch 'master' of github.com:ITCase/sqlalchemy_mptt into path-33
Flushing the session now expire the instance and it's children fixes #33
Hello 0.1.0 version