Fixed: Add @tstamp, @staff, and @layer to clef
Fixed: Add @tstamp, @staff, and @layer to clef

Previously the clef element was missing some information about it's exact positioning (e.g., for a clef change). This commit adds more information to the clef element to help reconstruct its position in the bar.

Fixes #27

annplaksin/sibmei,music-encoding/sibmei