Implement PID control of temperature

[brettski74]

Current code uses hard-coded PWM settings to try to follow a proper reflow profile, but these don't necessarily have any correlation to what the temperature is actually doing over time. A PID control strategy should allow for more accurate control of temperatures and rates of temperature rise/fall, provided that we have accurate temperature measurements to begin with. It should also allow the device to be able to be calibrated to work for different power supplies and plate designs.