Align Rests With Beams And Tuplets

beam.js :: align rests with beamed notes

dot.js :: change position of dots for dotted rests

tables.js :: add options to note glpph duration_codes for positioning
dots on dotted rests.

stavenote.js :: add function to determine rest position for beamed and
tuplet notes.

tuplet.js :: align rests with tuplet notes

vex.js :: add functions to find mid point between two lines

rests_tests :: new test for aligning rests with beamed and tuplet notes