Add manifests to the doc. Fix #27

A manifest is a JSON file describing a driver and its options.
It is mandatory for each driver.
Add LocalStorage's manifest
Add a launcher for the drivers. Store the manifest in the db.

The drivers don't have to implement '__main__.py' anymore, as it
is now common to all the drivers.

The drivers should now export 'start' and 'plug' in their
'__init__.py' file.

This launcher reads the manifest and give it to the Plug. The Plug
stores it in the database.
Update the doc to reflect the drivers' launcher changes.
Validate the options in the Plug using the manifest
Merge branch '#27_manifest' into develop