Fixed: sb elements are now encoded properly

Previously the system break element was only encoded if the "lock format" option was enabled. This commit outputs system breaks for all explicit system breaks.
Fixed: _convertBreak now uses constants
New: scoreDef carries page measurement information
Fixed: clef changes are now enclosed in staffDefs within a milestone scoreDef

Fixed #1
Fixed: encoding ties as both elements and attributes was redundant

Now ties are only encoded as attributes on /note elements.
Fixed: Milestone clefs are now encoded in layer

This fix moves clef changes from scoreDefs to milestone clef elements that occur within layer.

Refs #1
Fixed: Remove trace
Fixed: Hidden measure rests now have their visible flag set