Refactor persistence

Refactor the whole persistence system. Should be faster and more robust.
The main features are : 
+ objects are loaded by bulk by default
+ all children of a collection are loaded together, instead of loading each children per item.
(basically the loading and saving of an object hiearchy is done breadth-first instead of depth-first)
+ writing persistor for normal class is easier
+ associations have a corresponding object allowing dirty attributes.