Draw noteheads after stem

This pull request re-orders the drawing order in `StaveNote`so that the note heads are rendered on top of the stem instead of the other way around. This really only matters when the note/stem are different colors.

**OLD**
![image](https://f.cloud.github.com/assets/1631625/2478092/eab762a0-b079-11e3-89c1-b7002c39aef4.png)

**NEW**
![image](https://f.cloud.github.com/assets/1631625/2478085/c5dcab48-b079-11e3-8477-b1860820d3d9.png)