Added more robust TextDynamics

So I created a new object for more robust dynamics text. Basically you can create text dynamics that have any combination of the traditional letters: **p, f, m,z, r**

```
var notes = [
  new Vex.Flow.TextDynamics({ text: "sfz", duration: "4" }),
  new Vex.Flow.TextDynamics({ text: "rfz", duration: "4" }),
  new Vex.Flow.TextDynamics({ text: "mp", duration: "4" }),
  new Vex.Flow.TextDynamics({ text: "ppp", duration: "4" }),
  new Vex.Flow.TextDynamics({ text: "fff", duration: "4" }),
  new Vex.Flow.TextDynamics({ text: "mf", duration: "4" }),
  new Vex.Flow.TextDynamics({ text: "sff", duration: "4" })
];
```

![image](https://cloud.githubusercontent.com/assets/1631625/2885582/5e0a5ea8-d4c7-11e3-880c-b8d04121914a.png)

Obviously this overlaps with `TextNote` a bit, but provides more targeted functionality. Although, I'm not inheriting from `TextNote` and I'm not sure that's necessary. So let me know your thoughts.