Don't dispatch cancelAll calls.

- should fix pre-fetcher problems #247
- cancelAll should be always called using the main thread

SDWebImage/SDWebImageManager.m

@@ -129,12 +129,9 @@
 }
 
 - (void)cancelAll
-{
-    dispatch_async(dispatch_get_main_queue(), ^
 {
     [self.runningOperations makeObjectsPerformSelector:@selector(cancel)];
     [self.runningOperations removeAllObjects];
-    });
 }
 
 @end