Write metadata to extended attributes.

All filesystems, with the exception of FAT32 (and other ancient ones), support metadata. Chrome has been adding metadata to every file you download ("referrer.url" and "origin.url") for a while now! I'd love to see `youtube-dl` do the same. So, I've added this patch.

The patch takes the information in the `info_dict`, and updates the downloaded file's extended attributes using the `xattr` python module. The metadata format adheres to the [dublin core](http://dublincore.org/documents/usageguide/elements.shtml) and [XDG metadata](http://freedesktop.org/wiki/CommonExtendedAttributes/) standards.

Python module requirements: https://pypi.python.org/pypi/xattr

(Note: This patch doesn't change the user's experience in any way -- if the `xattr` module isn't installed, it does nothing, and if your filesystem doesn't support xattrs, it also does nothing.)