Added array like collections so changes to vertices can be observed.

Added a Collection to Two.Utils which inherits Backbone.Events and listens to:

* pop
* shift
* push
* unshift
* splice

Changes to the collection trigger ```'inserted'``` and ```'removed'``` events, passing an array of items that have changed.

I updated the internal vertices array to be a Collection, and bound the events to trigger ```updateVertices```

This enables creating a polygon and pushing / popping vertices to / from it.

```javascript
var path = new Two.Polygon([], false, true):
var vector = new Two.Vector(150, 150);
path.vertices.push(vector);
```