Parse Ma2 by parts

[donmai-me]

There are four parts (more for other games) for ma2: header, events, notes, and statistics. Each part is separated by an empty line. It would be nice if instead of parsing a chart line by line and ignoring certain parts, the parser would parse a chart part-by-part.

The header contains metadata about the chart and the resolutions used for timing. Events are a list of bpm and time signature changes in chronological order. Notes are self-explanatory. And, statistics contains information about all of the notes present in the chart.