Added StaveLine class, draws configurable lines between notes

It's pretty simple, originally started as something to support glissando notation, but I evolved it to be a highly configurable line/arrow class that would certainly be useful for educational diagrams.

![image](https://cloud.githubusercontent.com/assets/1631625/2768222/26d608cc-ca42-11e3-8eed-22b9fe322a58.png)

![image](https://cloud.githubusercontent.com/assets/1631625/2768225/2ced1aa2-ca42-11e3-862d-2feccba427e8.png)

Here are the `render_options` for the class
```javascript
this.render_options = {
    // Space to add to the left or the right
    padding_left: 4,
    padding_right: 3,

    // The width of the line in pixels
    line_width: 1,
    // An array of line/space lengths
    line_dash: null,
    // Can draw rounded line end, instead of a square
    rounded_end: true,
    // The color of the line and arrowheads
    color: null,

    // Flags to draw arrows on each end of the line
    draw_start_arrow: false,
    draw_end_arrow: false,

    // The length of the arrowhead sides
    arrowhead_length: 10,
    // The angle of the arrowhead
    arrowhead_angle: Math.PI / 8,

    // The position of the text
    text_position_vertical: StaveLine.TextVerticalPosition.TOP,
    text_justification: StaveLine.TextJustification.CENTER
  };
```