Fix auto-stem calculations

Automatic stem directions for beam groups should be correct now (this addresses the issues brought up in #118 ). The same function now used when `autoStem` is passed on `Beam` construction, and when we call `.generateBeams()`.

Tests have been added with checks on the stem direction for each beam. The auto-stem test has been renamed for clarity and now uses the same checks as the automatic beam generation since they should be identical.

**Auto-Stem on Beam Contruction**
![image](https://cloud.githubusercontent.com/assets/1631625/3897779/a5ff7e56-2264-11e4-95b6-60f495b5f201.png)

**Automatic Beam generation**
![image](https://cloud.githubusercontent.com/assets/1631625/3897799/cefb39a8-2264-11e4-92f2-aa2611110b7a.png)
![image](https://cloud.githubusercontent.com/assets/1631625/3897806/dc8e8638-2264-11e4-9ef1-106d6559a571.png)

Fixed a bug in `TabNote` where the stem direction was being forced up no matter what.