Starting issue for better memory mangment

Basically we need to extend the object classes and add few methods to keep local private information the previous state of the document, so at update time we can easily send a diff instead of saving back the whole object.

It can be done by adding some code at __destruct() that clean up everything (right now ActiveMongo2\Connection has a reference to all objects/documents and there is no way to free them up)
Improved memory

Now every document has its own information in memory, and they gone from RAM when the object is destroyed
minor change
minor change
Merge branch 'feature/better-memory' into develop