Bug fix - inconsistent line widths in Raphael

Noticed in the Raphael stave tests that lines are slightly thicker after the first stave is drawn:
![before_zoom](https://cloud.githubusercontent.com/assets/506171/3084167/29404f32-e4f0-11e3-8d33-7d21a9794b37.png)
Tracked it down to vex.flow.stem.draw().  Here's how it looks after the fix:
![after_zoom](https://cloud.githubusercontent.com/assets/506171/3084197/da82d332-e4f0-11e3-93ee-d8c16c1ef4d2.png)