Added microphone TX (very basic for now)

firmware/application/audio.hpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (C) 2014 Jared Boone, ShareBrained Technology, Inc.
+ * Copyright (C) 2016 Furrtek
  *
  * This file is part of PortaPack.
  *
@@ -38,6 +39,13 @@ void unmute();
 
 } /* namespace output */
 
+namespace input {
+
+void start();
+void stop();
+
+} /* namespace input */
+
 namespace headphone {
 
 volume_range_t volume_range();