Realized the performers pitch --> speed option

This replaces the old "relative durations" option. It
allows the performer to control the playback speed
for each NoteOn sent.
Bug Fix in Score.js: since notes can now tie over
barlines, voices no longer have to be the same length
in a system, so the checks were simply incorrect.
Improved pitch --> speed conversion

Also corrected the Variable Speed text on the main
page.
Further improvement to the pitch-->speed algorithm.