fix bug (global configure for downloader)

When used the SDWebImageManager class, we need configure the cache and downloader. In order to implement it, we need global configure.
Like this:
    SDWebImageDownloader.sharedDownloader.maxConcurrentDownloads = 12;
    SDWebImageDownloader.sharedDownloader.executionOrder = SDWebImageDownloaderLIFOExecutionOrder;
    ....
So that,  in anywhere we call the functions to set image, it can be delivered immediately.