Added support for full measure rests

Added ability to center-align a tickable and set a custom duration. Relevant Issue is #223. 

**Center Aligned**
```javascript
new Vex.Flow.StaveNote({
  keys: ["b/4"], 
  duration: "1r", 
  align_center: true
});
```
![image](https://cloud.githubusercontent.com/assets/1631625/2991652/674132be-dc81-11e3-8d84-960e290a62c3.png)

**Custom Duration**
```javascript
// Create custom duration
var custom_duration = new Vex.Flow.Fraction(3, 8); 
new Vex.Flow.StaveNote({
  keys: ["c/4"], 
  duration: "1r",
  duration_override: custom_duration
});
```
![image](https://cloud.githubusercontent.com/assets/1631625/3010003/9885d818-df0e-11e3-8f25-e8128bc4e9ce.png)

**Custom Duration + Center Aligned = Full Measure Rest**
```javascript
// Create custom duration
var custom_duration = new Vex.Flow.Fraction(3, 8); 
new Vex.Flow.StaveNote({
  keys: ["c/4"], 
  duration: "1r",
  duration_override: custom_duration,
  align_center: true
});
```
![image](https://cloud.githubusercontent.com/assets/1631625/2991654/72061e4e-dc81-11e3-8460-98b3b1fd1e1d.png)

Yeah I believe that would work! Definitely a preferable solution to having a conditional in every modifier.