Adding p5.sound module

The p5.sound module extends p5 with to work with sound. It uses the Web Audio API and shares syntax with the Processing Sound library (also currently under development). So far, it's based around the following classes:

*   SoundFile
*   Amplitude
*   FFT 
*   Oscillator (SinOsc, SqrOsc, SawOsc, TriOsc, Pulse)
*   AudioIn

More classes are coming soon, and I'm going to keep refining. But hopefully this is a viable starting point!

This includes the p5.sound module with inline documentation in /lib/addons, and examples in addons/p5.sound.