Fixes Issue:538 - Memory problem/leak on iOS 7

This was happening as the decompressed images take up lot of memory on older iOS devices
such as iPad 2 with a collection view and crashes the app without a memory warning most of the times.
These options don't break existing features but adds:
    * Adds Queue options to SDWebImagePrefetcher.
    * Adds option to decompress images in cache and post download.