Implement IFrameEncoder / IFrameDecoder using libvorbis

[gavv]

We have two interfaces IFrameEncoder and IFrameDecoder that are used to encode audio samples into packet payload and decode it back. Currently they're implemented using PcmEncoder and PcmDecoder (for uncompressed PCM).

Now we need to add two more implementations: VorbisEncoder and VorbisDecoder, that will use libvorbis library to do the job.

We also need to add new encoder & decoder to the list of tested codecs in test_frame_encoder_decoder.cpp.