Add AudioTransform: convert samples

* AudioTransform: does the same process as ColorTransform, but applied
to audio.
* AudioDesc: add a function to get a FrameDesc.
* AudioRewrapper: use AudioTransform.
InputStreamAudio: suppress tmp variable
AudioDesc: include AudioFrame instead of forward declaration

AudioFrameDesc can be used in the public API.
AudioTransform: set options in a simpler way

Use swr_alloc_set_opts() instead of setting options one by one, with a
risk of runtime error if an option we want to set doesn't exist.
AudioRewrapper: refactoring

* Rename audioODesc to audioOutputDesc.
* Explain the magick numbers ;)
Merge pull request #6 from cchampet/dev_audioProcessing

Add AudioTransform: convert samples