Feature: Add ability to un-cache a song

[eharris]

If an error occurs and iSub ends up caching a corrupted/incomplete song, there's no way to get it to play correctly because iSub thinks it's already cached and won't re-fetch it.

After a file is considered cached/downloaded, rather than the Download button (from long-press on the song) going inactive, it should change to something like "Remove" to be able to force remove it from the cache so it can be re-downloaded properly. Usually when I really want this feature it is due to #9, but I could see it being helpful other times too, like when iSub has cached a song that is very large (via the next track preload, on random) but that I don't want to waste cache space on, or when it has cached an incomplete file.