Added the ability to specify the key to be used when caching images

I've added the ability to specify the key that AFImageCache should use. I did this because while my base URL doesn't change, the parameters on the end of my URL do; even though the image itself doesn't change. Because AFImageCache uses url.absoluteString as a key, I end up with several versions of the same image all stored under a slightly different key.

The ability to specify an AFImageCache key also gives objects the flexibility to cache images using their unique id, or some other unique key based on the objects specific needs.