getNoteIndices(): improve comments
fix bug with handling of repeated chords (issue #47)

https://github.com/aspiers/ly2video/issues/47
fix sync breakage when MIDI starts with a chord (issue #48)

https://github.com/aspiers/ly2video/issues/48
safeRun(): handle KeyboardInterrupt more gracefully
rename measureTempoChanges -> secsElapsedForTempoChanges
secsElapsedForTempoChanges(): rename secsSinceIndex -> secsSinceStartIndex
initialise tempoIndex correctly during silent preludes (issue #48)

https://github.com/aspiers/ly2video/issues/48
test case for starting with a chord excluded from MIDI (issue #48)

https://github.com/aspiers/ly2video/issues/48