Added PauseMarking class

**Multiple Glyph Types**
![image](https://cloud.githubusercontent.com/assets/1631625/3032777/01f90d4e-e05c-11e3-996b-26d9cbab0f51.png)

**Custom Line Placement**
![image](https://cloud.githubusercontent.com/assets/1631625/3032785/1268d8da-e05c-11e3-9fd3-c55c29988416.png)

**With Articulation** *to handle caesura + fermata*
![image](https://cloud.githubusercontent.com/assets/1631625/3032792/2fcc11da-e05c-11e3-9f14-a9913ba3af71.png)

This clutters up `Articulation` a bit, so let me know what you think of the code. We've only dealt with articulations on `StemmableNotes` up to this point and these markings are stemless. So I had to do some checks to make sure to make `Articulation` didn't try and access stem data that didn't exist.
It's better to special case by functionality instead of by category. Since you're only using `is_pause` to change the y-offset I would prefer to have something like `this.note.hasFixedY()` which is what you check here. And you can get the actual offset values via something like `this.note.getFixedYs()` which might return `{top: 25, bottom: 30}`.

This will reduce the coupling between the two classes, and allow you to apply the same behavior to other note categories, if necessary.