Readme Changes and `addOperationWithBlock`

I have pull request that addresses:

1. A few typos in README.md;

2. Tweaked the code samples in README.md so that they would properly handle situations where the tableview in question had scrolled off the screen; and

3. Fixed `addOperationWithBlock` to just pass `block`, rather than doing the `dispatch_async` to a global queue. It is simpler and solves problems where the previous code might not correctly observe `maxConcurrentOperations`.