First version of a `copyright` file

While doing this licence audit, I realised there were some
headers which were wrong, so I added proper licences and
authors metadata here, and I also proposed a patch upstream
to fix the situation, which was committed in:

http://cgit.freedesktop.org/gstreamer/gstreamer-sharp/commit/?id=bcb8c7d1a311e4b30e4f8c4ce8d789a994f64d50