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

- Modified again the em_test to fit with testing purpose
- First test on multi datasource query written
- debugging of query.py

- adds tests on filter parse & check
 - standart filters parse & check
 - relational filters parse & check
- bugfixes related to tests in query.py
- update the example and the test EM
- bugfixes in admin script

Now a backreference is represented by a tuple(LeObject child class, fieldname)

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.