Merge pull request #32 from ITCase/develop

Update README.rst
Merge branch 'master' of github.com:ITCase/sqlalchemy_mptt
Merge branch 'master' of github.com:ITCase/sqlalchemy_mptt into level
test added
expire level
separate weak-ref set for each session
Merge pull request #36 from fayazkhan/failed_flushes

Fix concurrency issue with multiple session
Merge pull request #35 from fayazkhan/level

Category level