AKCallbackInstrument more usable
AKCallbackInstrument more usable

Changed callbacks into callback? :
- an array is unnecessary and less efficient.
- make it optional avoid some tricky things (egg-and-chicken dilemma
with callback definition and instrument init)

roecrew/AudioKit,narner/AudioKit,roecrew/AudioKit,narner/AudioKit,narner/AudioKit,roecrew/AudioKit,narner/AudioKit,roecrew/AudioKit