extracted opus to audio encoder/decoder interface available in lua

Ignore-this: 943c1a67fac3d38319b33abc655853a6

This allows apps to modify pcm data before encoding it. For example, you can
do audio effects or noise reduction.

darcs-hash:20140714192319-c21fe-a5fe4fb2a00d02d7958a8930d7f132524c9f88e6