[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/

React/Base/RCTUtils.h

@@ -61,10 +61,6 @@ RCT_EXTERN BOOL RCTImageHasAlpha(CGImageRef image);
 // Create an NSError in the NCTErrorDomain
 RCT_EXTERN NSError *RCTErrorWithMessage(NSString *message);
 
-// Dispatch an error + result callback on the main thread.
-typedef void (^RCTResultOrErrorBlock)(NSError *error, id result);
-RCT_EXTERN void RCTDispatchCallbackOnMainQueue(RCTResultOrErrorBlock callback, NSError *error, id result);
-
 // Convert nil values to NSNull, and vice-versa
 RCT_EXTERN id RCTNullIfNil(id value);
 RCT_EXTERN id RCTNilIfNull(id value);