- now we have a LodelHook to trigger migration handlers loading
- datasources initialization in dyncode is now trigger by a hook (lodel2_plugin_loaded)
- datasources loading by dyncode is better

- 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

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

- 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()