Tested and debugged with and without live performer

No known bugs.
assistantPerformer.html: defaults for Song Six changed.
In all files, moment.adjustedTimeReSequence has been
refactored to moment.msPositionReSubsequence.
Assistant.js:
playSpan() refactored to perform()
Assistant constructor now sets all
moment.msPositionReSubsequence attributes.
perform() sets all moment.timestamps to UNDEFINED_TIMESTAMP.
When handling noteOn input events, the speed factor is
now set *inside* handleNoteOn.
Sequence.js (Quite a lot of work done here!)
playSpan() refactored to play().
Fixes related to run() and resume(). Pausing now works
correctly...