Synthesize `executing` and `finished` properties.

It wouldn't build on Xcode 6 otherwise.

SDWebImage/SDWebImageDownloaderOperation.m

@@ -39,6 +39,9 @@
     BOOL responseFromCached;
 }
 
+@synthesize executing = _executing;
+@synthesize finished = _finished;
+
 - (id)initWithRequest:(NSURLRequest *)request options:(SDWebImageDownloaderOptions)options progress:(void (^)(NSInteger, NSInteger))progressBlock completed:(void (^)(UIImage *, NSData *, NSError *, BOOL))completedBlock cancelled:(void (^)())cancelBlock {
     if ((self = [super init])) {
         _request = request;