Correct order of tracks when playing multi-disc albums

SQL query output for PlayAlbum action is now ordered by iDisc first to prevent incorrect playlist sequence for albums which have more discs (1st track from 1st disc >>> 1st track from 2nd disc >>> 2nd track from 1st disc >>> 2nd track from 2nd disc >>> ... )
Merge pull request #20 from Kucheek/develop

Correct order of tracks when playing multi-disc albums
Thanks, Kucheek!