Return the value of this datahandler when empty

refs #131

refs #131

Methods are written.
They are not called by anyone and the only tests that were done is that no syntax error is raised.
( refs #131 )

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

Deleted hardcoded condition that brokes standart field validation process

Updated interface with nex dyncode

- datahandler base class was not supporting default values due to a bug
- implements the datetime datahandler construct_data method

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

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