Add symlink support (Fixes #42)

It's missing the option for relative symlinking, but this should hopefully do the trick for now. Merry Xmas!
I think so. Basically a vinyl file object is read in from the stream, a symlink is created pointing to it on disk, then the vinyl object is altered to point to the symlink and it's written back to the stream. 

I thought this behaviour would be the most similar to dest's behaviour.