Now passing UIImage as a placeholder

Changed the parameters of the loadImageFromLink method to include a UIImage instead of UIView.

There is no need to add a view to the UIImageView as we can easily take the placeholder image and set it as the image of the UIImageView. Once the new image was loaded, we simply setImage with the downloaded image.

The UIActivityIndicator will now appear in both cases until image was loaded or error occurred. 

Also, setting the placeholder *only if* image is not in cache.