[SlimDevices: Beta] Add time finished to Music Library Scan status report

[Phil Meyer]

>SqueezeCenter has finished scanning your music collection.
>The scan took <hour:mins> and was completed at
><hour:min:second:day:month:year> (in the correct format!).
>
Yes, this would be good as a confirmation that the scan has worked.  I don't think it's totally clear when a scan fails.  I raised bug 6457 recently about that.