Proposal for some `UIImageView (AFNetworking)` changes

Here I some changes I would like to propose to `UIImageView (AFNetworking)` which I think are reasonable:

* `-[UIImage setImageWithURL:placeholderImage:]` now tries to find a previous cached `NSHTTPURLResponse` and sends the value of the HTTP `Last-Modified` Header in the `If-Modified-Since` HTTP Header of the new request. The NSURL loading system doesn't do this on its own and that way, we allow the server to respond with a plain and lightweight `304` response code. The NSURL loading system will then automatically pick up the cached response on its own. Almost no network traffic is being generated in that case.
* `-[AFImageCache cachedImageForRequest:]` now tries to find a cached image in the shared `NSURLCache`.
* I changed the behavior of `-[UIImageView setImageWithURLRequest:placeholderImage:success:failure:]` in the following way:
    * By sending this message to an `UIImageView`, the developer basically tell the image view to set the corresponding image as soon as the data is available. __The default behavior, for me, would therefore be to set the image even if the user passes in a success block.__ Having to manage weak and strong reference for the image view is a pain in the neck if you want to set the image in the success callback.
    * By adopting this new behavior (`setting the image as soon as it is available`) and the improved image caching by utilizing 
 `NSURLCache` calling `-[UIImageView setImageWithURLRequest:placeholderImage:success:failure:]` will now try to first set the cached image from the memory cache. If it isn't found, it tries to find the cached image on disk thanks to `NSURLCache`. If that image is still not available, the placeholder image is being used.
    * `-[UIImageView setImageWithURLRequest:placeholderImage:success:failure:]` now always sends a request to the server. I think this is justified because we are sending the `If-Modified-Since` header. If the web server is smart enough to send back a `304` almost no traffic is being generated. The previous implementation had a small down site: If an image changed on the server side during the applications lifetime, the change would only be picked up if the `AFImageCache` has been evicted. This might get pretty frustrating for the actual user of the application if he expects an image to change but it doesn't.