More SDWebImageDownloader Cleanup

Remove async calls from SDWebImageDownloader.  The operation is already run in a background thread so the async calls seem unnecessary.

Couple notes about the change:

* NSNotifications are called on whatever the current thread is that is running the operation instead of the main thread

* The low priority support is removed - but the download is on a background thread so hopefully shouldn't matter

* Removed the queue that is no longer used

See what you think.