Audio support for the x264 frontend

by Kovensky for VideoLAN

Audio support on the x264 frontend. The plan is to, at the end of the project, have x264 capable of transcoding or copying audio as simply and correctly as possible. Usage should be as simple as 'x264 input -o output', with eventual support for {down,up}mixing and resampling. The used library for decoding and encoding will initially be libavcodec, but more libraries can be used in the future.