+ 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 :)
Compilers complain about comparing signed and unsigned here.  Replace int with `std::size_t`?