Show singles in the Year nodes in music db

ATM singles aren't shown by default in album listings where an artistid or artist isn't specified as part of the filter.  This makes a certain amount of sense, but in some cases we definitely want them.

This adds `singles=true` to the URL options parsed when an artistid or artist isn't specified, and allows the singles album item(s) to be returned.

@Montellese: Perhaps `allowsingles` might be a better name?  Any better ideas?  Initially I thought about trying to get rid of the restriction, but I fear that may cause more hassles - not sure.