Added TextBracket class

**Construction**
```javascript
var octave_bracket = new Vex.Flow.TextBracket({
  start: notes[0],
  stop: notes[1],
  text: "8",
  superscript: "vb",
  position: Vex.Flow.TextBracket.Positions.BOTTOM
});

```
**Render Options**
```javascript
this.render_options = {
  dashed: true,
  dash: [5],
  
  color: "black",
  line_width: 1,
    
  show_bracket: true,
  bracket_height: 8,

  // In the BOTTOM position, the bracket line can extend
  // under the superscript.
  underline_superscript: true
};
```
**Tests**
![image](https://cloud.githubusercontent.com/assets/1631625/3127232/76e6982a-e7b8-11e3-8842-ab6e64051775.png)
![image](https://cloud.githubusercontent.com/assets/1631625/3127240/be37e6ac-e7b8-11e3-9506-86159b32bda0.png)