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

- 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

To start in standalone uwsgi set lodel2.webui.standalone=True in conf file and start loader.py

- fixes exceptions output
- add "none" validator that validates only None