Support occupancy sensors

[benfrancis]

As a user I want to see the status of an occupancy sensor so I know whether there is a person in a room, even if they are not moving.

We currently support motion sensors, but not occupancy sensors which are slightly different, sometimes just more sensitive than a motion sensor.

[flatsiedatsie]

Perhaps useful: this one even has multiple ranges. Maybe it would be interesting to get show somee kind of range estimate in the capability icon?

[benfrancis]

OK, I think the first step is to define a capability schema for occupancy sensors (aka presence sensors) so I have filed WebThingsIO/schemas#83 for that.

We could potentially define a RangeProperty as an optional property of the OccupancySensor capability? Let's continue that discussion in the schema issue then track implementation of that schema in the gateway here. I'm also tracking implementation in the Zigbee adapter in WebThingsIO/zigbee-adapter#351.