[Image] Add onLoadStart, onLoadProgress, onLoadError events to Image component

Summary:
This PR adds 4 native events to NetworkImage.

![demo](http://zippy.gfycat.com/MelodicLawfulCaecilian.gif)

Using these events I could wrap `Image` component into something like:
```javascript
class NetworkImage extends React.Component {
  getInitialState() {
    return {
      downloading: false,
      progress: 0
    }
  }

  render() {
    var loader = this.state.downloading ?
      <View style={this.props.loaderStyles}>
        <ActivityIndicatorIOS animating={true} size={'large'} />
        <Text style={{color: '#bbb'}}>{this.state.progress}%</Text>
      </View>
      :
      null;

    return <Image source={this.props.source}
      onLoadStart={() => this.setState({downloading: true}) }
      onLoaded={() => this.setState({downloading: false}) }
      onLoadProgress={(e)=> this.setState({progress: Math.round(100 * e.nativeEvent.written / e.nativeEvent.total)});
      onLoadError={(e)=> {
        alert('the image cannot be downloaded because: ', JSON.stringify(e));
        this.setState({downloading: false});
      }}>
      {loader}
    </Image>
  }
}
```
Useful on slow connections and server errors.

There are dozen lines of Objective C, which I don't have experience with. There are neither specific tests nor documentation yet. And I do realize that you're already working right now on better `<Image/>` (pipeline, new asset management, etc.). So this is basically a proof concept of events for images, and if this idea is not completely wrong I could improve it or help somehow.

Closes https://github.com/facebook/react-native/pull/1318
Github Author: Dmitriy Loktev <unknownliveid@hotmail.com>

Libraries/Image/RCTImageDownloader.h

@@ -9,6 +9,8 @@
 
 #import <UIKit/UIKit.h>
 
+#import "RCTDownloadTaskWrapper.h"
+
 typedef void (^RCTDataDownloadBlock)(NSData *data, NSError *error);
 typedef void (^RCTImageDownloadBlock)(UIImage *image, NSError *error);
 
@@ -22,6 +24,7 @@ typedef void (^RCTImageDownloadBlock)(UIImage *image, NSError *error);
  * the main thread. Returns a token that can be used to cancel the download.
  */
 - (id)downloadDataForURL:(NSURL *)url
+           progressBlock:(RCTDataProgressBlock)progressBlock
                    block:(RCTDataDownloadBlock)block;
 
 /**
@@ -35,6 +38,7 @@ typedef void (^RCTImageDownloadBlock)(UIImage *image, NSError *error);
                     scale:(CGFloat)scale
                resizeMode:(UIViewContentMode)resizeMode
           backgroundColor:(UIColor *)backgroundColor
+            progressBlock:(RCTDataProgressBlock)progressBlock
                     block:(RCTImageDownloadBlock)block;
 
 /**