p5.sound: new classes and updates

- new classes: Env, LFO, Noise
- fade volume and frequency by providing 'fadeTime' as a second parameter
- tweaked the way SoundFile sets start/end points. SoundFile.cue() is now .jump(startTime, endTime). 
- gave Master output an additional gain node called 'meter'. Amplitude class now uses 'meter' when monitoring the amplitude of the entire sketch.
- changed SoundFile.playMode() syntax from 'mono' / 'poly' to 'restart' / 'sustain'
- adding freqToMidi and midiToFreq global helper methods
- reorganized and consolidated all the example soundfiles into a '_files' folder