…ction (read-on to understand why)

I finally found the reason behind the download not started while UITableView is manipulated: the default NSURLConnection runloop mode. Its default mode is NSEventTrackingRunLoopMode which is interrupted by UI events. Changing default NSURLConnection runloop mode to NSRunLoopCommonModes just fix this good old responsiveness issue.

I thus decided to replace the current NSOperation based implementation by this finding, as NSOperation is far more expensive than simple async connections. Additionally, moving aways from NSOperation here fix an odd lagging issue with iOS 4, an issue I can't explain at the moment.

Note that `SDWebImageDownloader`'s `setMaxConcurrentDownloads:` method is now a no-op as I didn't implemented the NSOperation queuing system with async connections. I don't think it still necessary as thread-less async connectaions are very lightweight. If you think there is a real need of this, I may reconsider and implement it in the future. In the meantime, this method does nothing and its usage is declared as deprecated.