Added new UIImageView+AFNetworking setImage method that takes a progress block.

Added this method:

```obj-c
- (void)setImageWithURLRequest:(NSURLRequest *)urlRequest
              placeholderImage:(UIImage *)placeholderImage
                       success:(void (^)(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image))success
                       failure:(void (^)(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error))failure
                       progress:(void (^)(float progress))progress;
```

A small addition that I found useful when downloading larger images.

What do you think?