Send a list containing "unkown" to summary.html if values are unkown

summary.html expects artist, title, release and track number to be
lists. If any of those tags is not a list, it will still access the
first element for printing, which, in the case of unkown values, was
just "[".
Merge pull request #25 from mineo/unkown-fix

Send a list containing "unkown" to summary.html if values are unkown