Add doorbell code

[jsiegenthaler]

The doorbell was requested in issue #143 so i thought I would add it for you as a stateless programmable switch.
It could also be anything else, such as a pushbutton, but the stateless programmable switch made moste sense in the Home app.

You can trigger the doorbell using standard webhooks commands for the stateless programmable switch:
http://localhost:51828/?accessoryId=doorbell1&event=0 // ring the doorbell using single press
http://localhost:51828/?accessoryId=doorbell1&event=1 // ring the doorbell using double press
http://localhost:51828/?accessoryId=doorbell1&event=2 // ring the doorbell using long press

When the doorbell is rung, it will send the standard HomeKit notification "Roomname doorbell rang", and it will trigger any actions added in the Home app to the stateless programmable switch

The code is based on HttpWebHookStatelessSwitchAccessory, but limited to one button only (as multiple buttons make no sense for a doorbell).

The doorbell does not trigger any urls to be called (but neither does HttpWebHookStatelessSwitchAccessory)

This is worthy of a new release, but wait a day or so, as I will update the readme comments and config.json for the stateless programmable switch as a separate PR (to make the readme more understandable)

[jsiegenthaler]

When you merge this, you can close issue #143

[jsiegenthaler]

Hold on a bit, I'm going to improve the code to read props before updating values

[jsiegenthaler]

Code improved invalid values are now logged as warnings