The exposes entry looks similar to a switch, except the type is a
"light" and it has `state` and `brightness`. Tested only on single
channel dimmers.
Also added a PercentageTranslator to translate between the HomeKit
`brightness`, which is a percentage, to/from an arbitrary numeric value
in Z2M.
There doesn't seem to be any distinction in Z2M between occupancy and
motion sensors, but HomeKit has separate types. Most of the sensors are
PIR, so they are technically motion sensors instead of occupancy
sensors. There are of course _real_ occupancy sensors like mmWave, but
we'll deal with those when we get there.
