Send out progress callbacks sooner.

- as soon as the NSURLConnection gets initialized
- helps to allocate and show progress indication sooner and prevent large delays without progress indication (for example when the server takes long to respond)
- estimated size is always 0 at this point, clients can use this to decide if they want to handle this callback or skip it

SDWebImage/SDWebImageDownloaderOperation.m

@@ -71,6 +71,10 @@
 
         if (self.connection)
         {
+			if (self.progressBlock)
+			{
+				self.progressBlock(0, 0);
+			}
             [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self];
         }
         else