Now lefactory populate a LeObject attribute named _child_classes

- Now returning automatic common field classname in fields method

Adding a field to all non abstract classes to store in DB the name of corresponding LeObject subclass

Warning, this commit is in a broken state. The em_test.py doesn't works yet
because it uses old relation handler (that doesn't exists anymore). The pickle file in
example is broken too

This implementation is broken because of relation data_handler. Backreference is a broken concept and relation data_handler too.