Generalized image decoding and resizing logic

Summary:
public

Standardises the image decoding logic for all image sources, meaning we get the benefits of efficient downscaling of images from all sources, not just ALAssets.

Reviewed By: javache

Differential Revision: D2647083

fb-gh-sync-id: e41456f838e4c6ab709b1c1523f651a86ff6e623

Examples/UIExplorer/ImageCapInsetsExample.js

@@ -46,6 +46,7 @@ var ImageCapInsetsExample = React.createClass({
           <Image
             source={require('image!story-background')}
             style={styles.storyBackground}
+            resizeMode={Image.resizeMode.stretch}
             capInsets={{left: 15, right: 15, bottom: 15, top: 15}}
           />
         </View>