tree_id expires after flushing fixes #33
++test
++test for #33
Merge branch 'develop' of github.com:ITCase/sqlalchemy_mptt into develop
Merge branch 'master' of github.com:ITCase/sqlalchemy_mptt into level
test added
separate weak-ref set for each session
Merge pull request #36 from fayazkhan/failed_flushes

Fix concurrency issue with multiple session