feature: sound system refactoring

Don't merge yet!

This PR+the branch on @damiannz's repo are the working place for the refactoring of the sound system. 

Features at the moment: 

* ofSoundBuffer a class to store sound buffers equivalent to ofPixels. might need a setFromExternalBuffer and be implemented with float array instead of vector so it can be used to pass the native buffer around without need for copies

* ofSoundFile: allows to read from sound files to sound buffers, would be good to add writting too, probably should inherit from ofFile

* ofBasicSoundPlayer: a sound player that can be used on any platform that supports ofSoundStream. it uses ofSoundFile to read sound files and outputs through any ofSoundStream

* ofSoundMixer: a simple sound mixer

* ofSoundUtils: a utility file for sound, like commonly used sound interpolations, panning algorithm...* ofSoundBuffer

Send a pull request to the __feature-ofSoundRefactoring__ branch on @damiannz's fork of oF to add things here.

Dev notes are [here](https://github.com/openframeworks/openFrameworks/wiki/Sound-programming-notes).