Add a GStreamer-based decoder module

GStreamer is a cross-platform multimedia processing
framework for audio/video postprocess/capture/
encode/decode/render.

This module is intended for Audio/Video decoding using
the available GStreamer plugins present in the system.
Currently this module supports only video decoding
and can be extended to audio decoding as well.
ZeroCopy is currently not supported and must be added.

Codecs currently supported are h264, mpeg4, vp8, mpeg2,
flashvideo, wmv1/2/3, vc1.

Signed-off-by: Vikram Fugro <vikram.fugro@gmail.com>
Signed-off-by: Jean-Baptiste <jb@videolan.org>