End secondary beams

Initial pull request for issue #171

>My initial implementation is very simple. I added a `StaveNote.end_secondary_beams` property, which you can set on construction. So to break up the group into groups of three, add the flag every third `StaveNote`.

>![image](https://f.cloud.github.com/assets/1631625/2433676/fcba409a-ad99-11e3-9d58-f1b6b4b70da4.png)

```
new Vex.Flow.StaveNote({ keys: ["c/4"], stem_direction: -1, duration: "16", end_secondary_beams: true}),
```

>Another option would be to store an array in the `Beam` which would contain indices of the notes to break on.

>Probably the best - and more complex - option is to set secondary beam groups in a way similar to the new auto-beaming, to automatically set secondary beam groups based on a rational or sequence of rationals.