Better duplicate detection and no-artist filenames

- fixes issue #55
previously, if the artist could not be ascertained, the file would be
called ' - video_title'.  now, that same thing would parse to
'video_title', with no leading '- '

- slightly better dupe detection
prior to caching or checking cache contents, characters are converted
to lower case.  This fixes observed cases where "Artist - Title' and
'ARTIST - Title' are seen as different songs (and both downloaded)