- 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

- separates LeGetQuery tests and LeFilteredQuery tests
- small bugfixes associated to tests

- 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

Doing so forbids operators like << == =! in* etc..
The tests were updated

- the re for filter spliting is modified to better handle operators
- added leading & trailing space to some operators

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