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

- now datasources are declared with two conf keys : read_only and identifier
- the old datasources plugin hook is now declared in lodel/plugin/core_hooks.py. This file is imported after lodel/plugin/hooks.py in the loader.py

- 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

Adding tests for the Plugin class

They have to implement a _activate() method in their __init__.py file

- 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

- I do not understand wy the set test for Settings was OK before...

+ 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

+ a new validator named 'string', it tries to cast the conf value using str()

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