Add compressor support #26

It's quite trivial to control a compressor using a Relay to represent the spike relay and a Switch representing the pressure switch. The primary benefits of this approach are more control over when the compressor runs.

However, the Pneumatic Control Module (PCM) introduced in 2015 replaces the spike relay and the solenoid breakout board: the compressor and pressure switch wire directly into the PCM. In this case, we want to introduce a new PneumaticControlModule interface (perhaps named PneumaticsModule??) analogous to our PowerPanel, with a hardware implementation class that delegates to the WPILib edu.wpi.first.wpilibj.Compressor class. The interface should support closed-loop control.