Support sending beacon messages to EV3 IR sensor

[gregretkowski]

The EV3 IR Sensor/Remote has a Beacon mode where the Remote will send a constant beacon signal and the Sensor can detect the distance and direction to the beacon. This PR adds the ability to send the beacon signal from the PowerFunctions library.

Usage:

#include "PowerFunctions.h"

PowerFunctions pf(3, 0);

void loop() {
    delay(220);
    pf.send_beacon();
}