InputStreamAudio: fix compile error with libAv

AVFrame has no "channels" attribute in LibAv. We should use
"av_get_channel_layout_nb_channels" on the "channel_layouts".
InputStreamVideo: throw if an error occured during video decoding
InputStreamAudio: fix decode

* Use av_samples_copy instead of av_samples_fill_arrays to copy the
frame data to our DataBuffer.
* Get nb_channels by the CodecContext, intead of
av_get_channel_layout_nb_channels (see commit
1cdebe6c5c4e188d97a7921965bd8d22123b4e09).
* Throw a more specific runtime_error if an error occured during audio
decoding.
Audio: encode outputStreamAudio

* OutputStreamAudio:
	* define setup to open codec.
	* define encodeFrame.
* AudioFrame: add nbSamples attribute to get the number of samples to
transcode.
* InputStreamAudio:
	* delete print.
	* set number of samples decoded in the AudioFrame ; get this
value during encode.
* Update Rewrapper: add encode and wrap process.
OutputStreamAudio: overlay encodeFrame

* This new function gets delayed encoded frames.
* Rewrapper: call this new function.
Merge pull request #5 from cchampet/dev_audioProcessing

Audio: encode process