More instantiation options

Return this on .set and also add a view option so I can do this:

```javascript
var mySwitcher = new ViewSwitcher(this.getByRole('cat')).set(catSubview);
```

or this (according to preference, it's easy to support both)

```javascript
var mySwitcher = new ViewSwitcher(this.getByRole('cat'), {view: catSubview});
```