No need of -d flag to specifiy directories

Created a new class LodelSession to handle session datas
Created an automatic wrapper to acces to session handler plugin function throug SessionHandlerPlugin class methods
Bugfixes done for plugins. The plugin type is store in __plugin_type___ and not in __type__

Import bugfixes in settings validator. Hooks must be imported in validatore definition to avoid circular import failure

Implements both Plugin child classes handling extensions and session handling.
Modified the settings plugin preload piece of code. It is now adapted to the new plugins organisation.
Each Plugin child classes provides a confspec indicating where the plugin list is stored in configuration.

- now plugin is able to instanciate using the good child class
- the discover cache now contains plugin's type

- DatasourcePlugin implmentation as child class of lodel.plugin.plugins.Plugin
- moved methods about datasource initialisation from LeObject to DatasourcePlugin ( _init_datasource(), plugin_name(), _get_ds_connection_conf() ...)

- add a -L options to list availables actions

- Allow to extend easily lodel_admin.py script.
- Writtent first lodel_admin.py action : discover-plugins