Adding audio model classes

-audioManager : general object that can listen for ChordsEvent and play
them by building a virtual instrument from a soundGraph
-ChordsEvent : an object that represente a Chords
-ChordsEmitter/ChordsListener : interface for observer pattern
-SoundGraph/SoundNode : object that represente the actual instrument
(completely dissociated from minim) they are used to build  minim's
object by the audioManager
-VirtualInstrument : A minim Instrument that can use a soundgraph to
patch his components (oscil, adsr...)