Ticket #19: Automatically load and unload modules when needed.

Lustre modules are loaded and unloaded by Shine, once per server,
before doing appropriate other Actions like start or stop.

- Add a generic class for all server specific action:
    Shine.Lustre.Actions.ServerAction
- Add the two first action uses ServerAction: LoadModule and
UnloadModule in Shine.Lustre.Actions.Modules
- start, stop, mount and umount uses ActionGroup and ModuleAction.
- Add tests for the actions.