Support alchemy 0 9 plus

This branch port the library to SQLAlchemy 0.9+. It should have a different version number (tag, let's say 0.6 if we are using Semantic Versionning) so that users can pick the library depending on their SQLAlchemy version. There is a 'hack', i am not very proud of it, but I have no idea how to get past this (line 244).