ADD External Audio Device sample application

This application demonstrates how a developer might use the Audio Driver API
in version 2.3.0 of the OpenTok iOS SDK. It operates just as the Hello World
exmaple does, with an additional call to set the Audio Driver and an
implementation file for the Audio Driver itself.
MINOR Code cleanup and formatting
Merge pull request #45 from wobbals/audio-sample

Add an Audio Driver sample