0.4.0

• Adds support for react-native 0.8.0
• Adds support for custom annotation images
  • API looks like:

{
  latitude: 40.714541341726175,
  longitude:  -74.00579452514648,
  title: 'Important!',
  subtitle: 'Neat, this is a custom annotation image',
  annotationImage: {
    url: 'https://cldup.com/7NLZklp8zS.png',
    height: 25,
    width: 25
  },
  id: 'marker2'
}