Merged RCTNetworkImageView functionality into RCTStaticImage

Summary:
RCTNetworkImageView and RCTStaticImage had significant overlap in functionality, but each had a different subset of features and bugs.

This diff merges most of the functionality of RCTNetworkImageView into RCTStaticImage, eliminating some bugs in the former, such as constant redrawing when properties were changed.

I've also removed the onLoadAbort event for now (as it wasn't implemented), and renamed the other events to match the web specs for `<img>` and XHMLHttpRequest. The API is essentially what Adobe proposed here: http://blogs.adobe.com/webplatform/2012/01/13/html5-image-progress-events/

The following features have not yet been ported from RCTNetworkImageView:

- Background color compositing. It's not clear that this adds much value and it increases memory consumption, etc.
- Image request cancelling when images are removed from view. Again, it's not clear if this is a huge benefit, but if it is it should be combined with other optimisations, such as unloading offscreen images.

(Note that this only affects the open source fork. For now, internal apps will still use FBNetworkImageView for remote images.)

Examples/UIExplorer/ImageExample.js

@@ -32,7 +32,7 @@ var NetworkImageExample = React.createClass({
   getInitialState: function() {
     return {
       error: false,
-      loading: true,
+      loading: false,
       progress: 0
     };
   },
@@ -47,10 +47,10 @@ var NetworkImageExample = React.createClass({
       <Image
         source={this.props.source}
         style={[styles.base, {overflow: 'visible'}]}
-        onLoadError={(e) => this.setState({error: e.nativeEvent.error})}
-        onLoadProgress={(e) => this.setState({progress: Math.max(0, Math.round(100 * e.nativeEvent.written / e.nativeEvent.total))}) }
-        onLoadEnd={() => this.setState({loading: false, error: false})}
-        onLoadAbort={() => this.setState({error: 'Loading has aborted'})} >
+        onLoadStart={(e) => this.setState({loading: true})}
+        onError={(e) => this.setState({error: e.nativeEvent.error, loading: false})}
+        onProgress={(e) => this.setState({progress: Math.round(100 * e.nativeEvent.loaded / e.nativeEvent.total)})}
+        onLoad={() => this.setState({loading: false, error: false})}>
         {loader}
       </Image>;
   }