Added PID community library

Hi folks,

I have published a barely modified version of Brett Beauregard’s Arduino PID Library as a Spark community library, creatively named “PID”.

The only change from the original library is that I replaced the defines (AUTOMATIC, MANUAL, DIRECT, REVERSE) with namespaced enums (PID::AUTOMATIC, PID::MANUAL, PID::DIRECT, PID::REVERSE) to avoid naming conflicts.

Feedback appreciated.