+ ofSoundBuffer and ofSoundObject

This PR adds ofSoundBuffer (a few-frills container for a buffer of floats) and ofSoundObject (an API for connecting sound producing / consuming objects which pass ofSoundBuffers around).

It also integrates ofSoundBuffer into ofSoundStream, implementing the new buffer-based callbacks for the RTAudio and iOS sound streams.

This has been tested on OSX (Yosemite, MacBook Pro) and iOS (8.1, iPhone 5s, iPad 4). I'll see what I can do to test on Windows ASAP.

Ping @roymacdonald, @arturoc, @mazbox

This PR is part of an effort to chop up #2626 into more manageable pieces, and get some momentum going :)
Would it make sense to return references rather than pointers?  My students (and other new users) often get confused when a pointer is returned because there is a bit of confusion about the ownership.  Since we use returns by reference in the `ofPixel_<*>` classes, perhaps we could take the same approach with the sound objects?  

In this case, we'd probably also need `const ofBaseSoundInput& getInput() const` methods as well.