Add Aviator.rewriteRouteTo

Add simple helper for route descriptions so you can write things like
this:

```javascript
Aviator.setRoutes({
  '/foo': {
    '/bar': Aviator.rewriteRouteTo('/baz')
  },
  '/baz': {
    target: bazTarget,
    '/': 'index'
  }
});
```

This fixes https://github.com/swipely/aviator/issues/34

@barnabyc @flahertyb