[ASDKgram example update] addressed PR comments

- overrode the -(void)fetchData method in PhotoCellNode.m to download the photo’s comments. This method gets called with the PhotoCellNode enters ASInterfaceStateFetchData, which is set by the rangeController.
    - UIKIT COMPARISON: I left the comment bulk download (for all photos in a page load) in the PhotoFeedViewController side for UIKit because when implemented in the PhotoTableViewCell, each cell jumped around as it changed size when it came on screen.
- minor appearance updates
    - updated color scheme
    - fixed status bar style to darkBackgroundColor
- cleaned up layoutSpecThatFits: in PhotoCellNode

examples/ASDKgram/Sample/UserModel.h

@@ -8,7 +8,7 @@
 
 @interface UserModel : NSObject
 
-@property (nonatomic, assign, readonly) NSDictionary *dictionaryRepresentation;
+@property (nonatomic, strong, readonly) NSDictionary *dictionaryRepresentation;
 @property (nonatomic, assign, readonly) NSUInteger   userID;
 @property (nonatomic, strong, readonly) NSString     *username;
 @property (nonatomic, strong, readonly) NSString     *firstName;