Web application support for accessories - start.

Layouts can now manage a list of 'accessories' (turnouts, slips, etc). Those
are saved in the database, and displayed on the layout image on the main screen.
accessories can be dragged to their correct location on the layout image, and that
location is saved in the database too.
Implement turnout control on Arduino sketch and further work on web app

Added SPI support on Arduino sketch, along with set/query commands to
control turnouts. Not fully tested yet. Further work on web app to support
selecting the turnout/accessory address based on actual number of addresses
supported on the controller.
Bugfixing arduino sketch

Latch values were wrong earlier.
Server and app side accessory control

Web app sends commands to the controller to pulse accessories. Works
on prototype fairly well. Next step is: accessory icons, and more
layout edition options. Also distinguish between temporary pulse accessories
such as turnouts, and long pulse accessories such as uncouplers.