The UniqID datahandler has to call the datasource
	LeObject has to be able to determine wich parent class define its UID datahandler
	[NOT IMPLEMENTED] the datasource should be able to select with an abstract LeObject as target ( see #92 )

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

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