fix uninitialized value, fix unsigned calculation

[nekowokaburu]

Thanks for the lib! I noticed that the Relay version has some bugs:

• _lastPulseTime was uninitialized, I set it to millis() to catch up quicker
• the _lastPulseTime computation in the while loop can go wrong on two unsigned values, especially with one uninitialized ;)

I also moved the millis() check out of the loop as I'd rather have a more robust PID that can always catch up to the millis() value in the while loop than the small speed improvement, if any. I am running an espresso machine with this and would hate to see it blow up ;)