Allow MapScreen annotations to be images

A very simple change that allows the use of `image` param in the MapScreen `annotation_data`

```ruby
 def annotation_data
    [{
      longitude: -82.965972900391,
      latitude: 35.090648651123,
      title: "Rainbow Falls",
      subtitle: "Nantahala National Forest",
      image: "custom-pin".uiimage
    },{
      longitude: -82.966093558105,
      latitude: 35.092520895652,
      title: "Turtleback Falls",
      subtitle: "Nantahala National Forest",
      image: "custom-pin".uiimage
    }]
end
```