Added some comments

z2m.go

@@ -91,10 +91,14 @@ func (zDev *Z2MDevice) Publish(topic string, payload interface{}) {
 	}
 }
 
+// Check if the device is available, Z2M will send an availability message
+// if this enable in Z2M config
 func (zDev *Z2MDevice) Available() {
 	zDev.Publish("availability", `{"state": "online"}`)
 }
 
+// Ask for a device to dump its state. Z2M doesn't support this but the
+// dump-state-ext.js provide this feature
 func (zDev *Z2MDevice) Sync() {
 	zDev.Publish("dump", "{}")
 }