Partial beam bug fix

The beaming algorithm didn't take into account partial beams that fall on the left side.

![image](https://f.cloud.github.com/assets/1631625/2481048/bdd95138-b0d0-11e3-8394-c4c199a0a818.png)

Now fixed *(not identical notes as previous example)*: 

![image](https://f.cloud.github.com/assets/1631625/2481092/cfe14d08-b0d1-11e3-9b03-a52b56f773ec.png)