1.7.0 - Create note events at specific ticks

• New startTick param available for MidiWriter.NoteEvent() which can be used to place notes at a specific tick.
• Writer() constructor now accepts a single track or an array of tracks (no need to wrap single track in an array anymore).
• New Track.mergeTrack() method provides ability combine events from two tracks into one (still slightly experimental).
• General code organization improvements.