Fixed default rest positioning

This PR does two things:
1. Fixes positioning of Whole Rest
2. Normalizes rest positioning across all clefs while using the key type `R`, ie: `keys: ["r/4"]`. *(But really the octave is irrelevant, we just need it to not break key parsing)*

Basically, in `StaveNote.calculateKeyProps()` I simply override the `line` value if the `props.key` is `"R"`. Let me know if you'd prefer a different way of fixing this.

I also modified the `stavenote_tests.js`, so all rests simply take `r/n`. 

**Treble**
![image](https://cloud.githubusercontent.com/assets/1631625/3235773/383ffc0c-f0d6-11e3-8dbf-708df325aa4b.png)
**Bass**
![image](https://cloud.githubusercontent.com/assets/1631625/3235774/48b80912-f0d6-11e3-9e4b-d9477071a4b1.png)