Add support for dimmers and dimmable bulbs

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.

s_light.go

@@ -0,0 +1,50 @@
+package hapz2m
+
+import (
+	"github.com/brutella/hap/accessory"
+	"github.com/brutella/hap/characteristic"
+	"github.com/brutella/hap/service"
+)
+
+func createLightServices(dev *Device) (byte, []*service.S, []*ExposeMapping, error) {
+	var svcs []*service.S
+	var exposes []*ExposeMapping
+
+	for _, exp := range dev.Definition.Exposes {
+		if exp.Ignored() {
+			continue
+		}
+
+		switch {
+		case exp.Type == "light" && len(exp.Features) > 0:
+			exp := exp // create a copy
+
+			light := service.NewLightbulb()
+
+			for _, feat := range exp.Features {
+				if !feat.IsStateSetGet() {
+					continue
+				}
+
+				feat := feat
+
+				switch {
+				case feat.Name == "state" && feat.Type == "binary":
+					svcs = append(svcs, light.S)
+					exposes = append(exposes, &ExposeMapping{&feat, light.On.C, nil})
+
+				case feat.Name == "brightness" && feat.Type == "numeric":
+					brightness := characteristic.NewBrightness()
+					light.AddC(brightness.C)
+					exposes = append(exposes, &ExposeMapping{&feat, brightness.C, nil})
+				}
+			}
+		}
+	}
+
+	return accessory.TypeLightbulb, svcs, exposes, nil
+}
+
+func init() {
+	RegisterCreateServiceHandler(createLightServices)
+}