Add imageKey prop in order to help control the Image cache.

Ran into a similar problem as this issue: facebook/react-native#1397

Added an imageKey prop to facilitate here and removed the existing key prop as it was unnecessary.

Add Progressive and Placeholder Image Support

Add a progressive image with a prop to your component:

placeholderSource={{
  uri: 'http://progressive.image/yours.png'
}}

Note that the animationStyle will then be set to fade.

Placeholder images are similar with the placeholderSource prop:

placeholderSource={require('./path/to/image.png')}

Add Loading Animation to Placeholder

Added a simple animation to the placeholder that lightens its color by 20 points while loading.