Avoid useless impossible import by putting confpsec in a dedicated file

Now it support -v and -f options (kind of dirty support for the moment...)

- 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

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

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

see README.md for more informations about instance creation