p5.sound update

Additions:
- Record with p5.SoundRecorder. SoundFile has a .saveSound() method to save as .wav
- p5.Signal class introduced to manage .add, .mult and .scale of audio signals for modulation. p5.Oscillator and p5.Env now have .add, .mult and .scale methods.
- new way to modulate: pass a p5.Oscillator, p5.Env or p5.Signal as the parameter, i.e. carrier.freq(mod), or carrier.freq(mod.scale(-1, 1, 400, 600))

Changes: 
- p5.Oscillator defaults to 0 amp (better for envelopes)
- osc.start(time, freq) instead of osc.start(freq, time)
removing the .mod method for oscillator
- improvements to p5.Env, p5.Oscillator and p5.Pulse
- cleanup, documentation, examples