Beam groups

This isn't quite ready to merge. I'm not really sure about my implementation of `getDefaultBeamGroups`.

What are your thoughts of having a "Time Signature Default Beam Group" table? If you look at my implementation, it first looks for a hard-corded default, and if unsuccessful, guesses off of the meter. But I think I need to do some more research on exact beam rules. Or, at least, what is used often enough to be considered a default.

I also solved a bug where >= quarter note triplets caused weird beam groups.