Support for tagging videos and PDFs

[zadr]

We should be able to add tags for the track and the platform. The colors are undocumented-by-Apple[1] xattr values on _kMDItemUserTags, though, we should also maybe consider using words.

It would be nifty if WWDCDownloader was smart enough to add attributes to files that are already downloaded, not just to new downloads.

1. See answers by beroe and Sperr on http://stackoverflow.com/questions/19720376/osx-mavericks-add-tags-programmatically/19720455