Allow uploading a native file (e.g. photo) via XMLHttpRequest

Summary:
With this in place, it's possible to upload a picture from the `CameraRoll` to Parse, for instance:

    xhr = new XMLHttpRequest();
    xhr.onload = function() {
      data = JSON.parse(xhr.responseText);
      var parseFile = new Parse.File(data.name);
      parseFile._url = data.url;
      callback(parseFile);
    };
    xhr.setRequestHeader('X-Parse-Application-Id', appID);
    xhr.setRequestHeader('X-Parse-JavaScript-Key', appKey);
    xhr.open('POST', 'https://api.parse.com/1/files/image.jpg');
    // assetURI as provided e.g. by the CameraRoll API
    xhr.send(new NativeFile(assetURI));

Closes https://github.com/facebook/react-native/pull/1357
Github Author: Philipp von Weitershausen <philikon@fb.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/Image/RCTImageLoader.m

@@ -154,4 +154,11 @@ static void RCTDispatchCallbackOnMainQueue(void (^callback)(NSError *, id), NSEr
   }
 }
 
++ (BOOL)isSystemImageURI:(NSString *)uri
+{
+  return uri != nil && (
+   [uri hasPrefix:@"assets-library"] ||
+   [uri hasPrefix:@"ph://"]);
+}
+
 @end