[ReactNative] Refactor RCTDataManager to support pluggable data source modules (RCTURLRequestHandlers)

Summary:
@public

This is a refactor of @philikon's original diff that decouples the dependencies between the Network and Image modules, and replaces RCTDataQueryExecutor with a more useful abstraction.

I've introduced the RCTURLRequestHandler protocol, which is a new type of bridge module used for loading data using an NSURLRequest. RCTURLRequestHandlers can be registered using RCT_EXPORT_MODULE() and are then available at runtime for use by the RCTDataManager, which will automatically select the appropriate handler for a given request based on the handler's self-reported capabilities.

The currently implemented handlers are:

- RCTHTTPRequestHandler - the standard open source HTTP request handler that uses NSURLSession
- RKHTTPRequestHandler - the internal FB HTTP request handler that uses FBNetworking
- RCTImageRequestHandler - a handler for loading local images from the iOS asset-library

Depends on D2108193

Test Plan:
- Internal apps still work
- OSS port still compiles, Movies app and a sample Parse app still work
- uploading image to Parse using the above code snippet works
- tested `FormData` with string and image parameters using http://www.posttestserver.com/

Libraries/Image/RCTImageRequestHandler.m

@@ -0,0 +1,62 @@
+//
+//  RCTImageRequestHandler.m
+//  RCTImage
+//
+//  Created by Nick Lockwood on 09/06/2015.
+//  Copyright (c) 2015 Facebook. All rights reserved.
+//
+
+#import "RCTImageRequestHandler.h"
+
+#import <UIKit/UIKit.h>
+
+#import "RCTImageLoader.h"
+#import "RCTUtils.h"
+
+@implementation RCTImageRequestHandler
+{
+  NSInteger _currentToken;
+}
+
+RCT_EXPORT_MODULE()
+
+- (BOOL)canHandleRequest:(NSURLRequest *)request
+{
+  return [@[@"assets-library", @"ph"] containsObject:[request.URL.scheme lowercaseString]];
+}
+
+- (id)sendRequest:(NSURLRequest *)request
+     withDelegate:(id<RCTURLRequestDelegate>)delegate
+{
+  NSNumber *requestToken = @(++_currentToken);
+  NSString *URLString = [request.URL absoluteString];
+  [RCTImageLoader loadImageWithTag:URLString callback:^(NSError *error, UIImage *image) {
+    if (error) {
+      [delegate URLRequest:requestToken didCompleteWithError:error];
+      return;
+    }
+
+    NSString *mimeType = nil;
+    NSData *imageData = nil;
+    if (RCTImageHasAlpha(image.CGImage)) {
+      mimeType = @"image/png";
+      imageData = UIImagePNGRepresentation(image);
+    } else {
+      mimeType = @"image/jpeg";
+      imageData = UIImageJPEGRepresentation(image, 1.0);
+    }
+
+    NSURLResponse *response = [[NSURLResponse alloc] initWithURL:request.URL
+                                                        MIMEType:mimeType
+                                           expectedContentLength:imageData.length
+                                                textEncodingName:nil];
+
+    [delegate URLRequest:requestToken didReceiveResponse:response];
+    [delegate URLRequest:requestToken didReceiveData:imageData];
+    [delegate URLRequest:requestToken didCompleteWithError:nil];
+  }];
+
+  return requestToken;
+}
+
+@end