Add Opus support to neteq_rtpplay

BUG=2416
R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/30859004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7595 4adac7df-926f-26a2-2b94-8c16560cd09d