Test passes

- Written a new custom_list validator, it takes another validator name as argument (in order to validate items of list)
- Using it to validate list of plugins names

Implements the check of the bases classes of a datasource class in a datasource plugin ( solves #144 )

refs #132

- The validator is written and added to all Plugin sublcasses confspec
- Adds the capabilities to give args to validator (here we give the expected type)

Created groups (they can be found under the "Modules" menu -_- )
Created new pages : on LeAPI EditorialModel Plugins etc

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__

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() ...)