Break secondary beams - new pull request

Works like this:

```javascript
beam.setSecondaryBeamBreakIndices([1, 3]); // renders groups of two in a 6 note beam
```
Also, it looks like I accidentally committed the secondary bream break test in the grace note branch. But it's been updated now to work with this new implementation.

![image](https://f.cloud.github.com/assets/1631625/2455463/78ad037a-aeff-11e3-93b6-d5c4ddd11517.png)