Tweak UITableView examples to properly cell reuse

The previous code samples did not properly handle situation where the cell had scrolled off the screen by the time the asynchronous loading of the image was done. Likewise, the image wasn't being initialized (so that any residual image would be removed from the reused cell). Finally, the `dispatch_sync` can be `dispatch_async` ... no reason to keep background queue waiting.
Eliminate blank lines between comments and code

Made comment of code change more consistent with the rest of the code.
Simplified `addOperationWithBlock`

Simplified `addOperationWithBlock` to eliminate problematic `dispatch_async` reference.
Fix typos
Merge pull request #4 from robertmryan/master

Readme Changes and `addOperationWithBlock`