Write metadata to extended attributes, take two...

This is an updated version of my previous pull request, #1517, where I've merged all the changes into one big commit, so you can actually read it.

I also improved it a bit -- it'll try using the `setfattr` commandline utility if the `xattr` module isn't installed.

(I'm still waiting for feedback on how to get the `webpage_url` into the `info_dict` for all the extractors.)