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.
Remove this and add a PauseMarking.CATEGORY = "pausemarkings". (See other modifiers for examples.)