Implemented BPMTimeline.beat. Fixed a typo in BPMTimeline.add_bpm_marker. Implemented BPMTimeline.bpm_at_beat.
Implemented BPMTimeline.beat. Fixed a typo in BPMTimeline.add_bpm_marker. Implemented BPMTimeline.bpm_at_beat.

echo66/bpm-timeline.js,echo66/bpm-timeline.js