all: introduce a temperature type

This type should be used whenever a sensor (or actuator?) works with a
temperature. For example, this commit changes the signature:

    ReadTemperature() (int32, error)

to the following:

    ReadTemperature() (drivers.Temperature, error)

I believe this is much clearer in intent. It also makes it trivial to
introduce common conversions. For example, there are already Celsius()
and Fahrenheit() methods to convert to the given units, as a floating
point. More units could be added as needed, for example a CelsiusInt().

examples/bme280/main.go

@@ -22,7 +22,7 @@ func main() {
 
 	for {
 		temp, _ := sensor.ReadTemperature()
-		println("Temperature:", strconv.FormatFloat(float64(temp)/1000, 'f', 2, 64), "°C")
+		println("Temperature:", strconv.FormatFloat(float64(temp.Celsius()), 'f', 2, 64), "°C")
 		press, _ := sensor.ReadPressure()
 		println("Pressure:", strconv.FormatFloat(float64(press)/100000, 'f', 2, 64), "hPa")
 		hum, _ := sensor.ReadHumidity()