Test passes

In construct and check data

Return the value of this datahandler when empty

refs #131

- deletion of unused attributes
- copy the back_reference checks in _set_back_reference function and use it in Reference.__init__()

Updated interface with nex dyncode

- 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

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