FIX: work harder to find a valid duration for the music stream

Use xbmc.Player() duration information if a valid duration is not found in the MusicInfoTag

* NOTE: This change allows scrobbling of many more audio streams that may not be
  able to calculate & set a duration prior to handing off to the xbmc.Player

@ronie, I hope you'll accept this change as it helps music/audio plugin authors access the scrobbling mechanism even when they can't determine the stream duration before handing it off to the xbmc.Player. Plugins like Pandora (see https://github.com/rivy/xbmc-pandora) currently have to use hacks like setting a fictional duration in order to use the scrobbling mechanism.

I'll be submitting a similar pull request for the libre.fm scrobbler in the near future.

Thanks for your attention.