- now _datasource attr contains an instance of the datasource, instanciated with options given in conf file

+ better log message for datasource bootstrap

- now datasources are handled in LeObject child classes
- plugins module now have a method to retrieve a plugin module

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

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