Adding a field to all non abstract classes to store in DB the name of corresponding LeObject subclass

- Now delete & update methods use filters & rel_filters

- deleting most of **datas to replace it with a standart dict argument
- using filters in childs of LeFilteredQuery

- Added missing allowed_classes options to some references in em_test

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

- adding an EmClass to make a double link between indexes and texts
- adding subqueries infos to dump_infos method of LeFilteredQuery

- The set_query_filter() method handles subquery creation
- The run isn't written yet...

The _datasource_name attribute replace the old _datasource attribute. The goal of the modification is to maintain easyly accessible datasource UID

- renamed Plugins class to Plugin
- an instance represent a loaded plugin
- classmethod allows to preload & load plugins

- 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