fix bugs

1. fix the bug about collection_mapper when session init in session.py
and add get_sesion method
2. fix the bug in README.md for import apis
3. add attribute '__persistence__' in BaseModel to forbid redundant
update.
4. fix bug that Queue.get_all method not need params. queue.py
Merge pull request #3 from yeyuexia/fix/bugs

fix bugs