AudioIO Initial Release

This is the initial release of AudioIO, for simple audio IO in Julia. Currently it supports playing vectors as audio through the sound card using PortAudio, and automatically handles vectors of different types (UInt, Int, Float, in various bit depths).

There's also a more generic AudioNode framework for connecting together nodes in a render graph, which forms the backbone of the simple API (e.g. `play`, `stop`, etc.)

I've been in touch with @zhemao (author of Bebop.jl) and we plan to merge his libsndfile bindings into this project.