Refactor audio output to make it more modular.

This makes the player less hard coded to portaudio, and easier to
experiment with different backends.

src/lib.in.rs

@@ -2,6 +2,7 @@
 mod audio_decrypt;
 mod audio_file;
 mod audio_key;
+pub mod audio_sink;
 pub mod authentication;
 mod connection;
 mod diffie_hellman;