Added setImageWithURL:placeholderImage:success:failure: to UIImageView+AFNetworking

I had the need for this intermediate method several times now. 
If I just want to specify a URL and a success block I'm forced to build my URLRequest by hand, and I have to remember to put the relevant HTTP header in said request, because the next success-block taking method only works with URLRequests and not URLs.

This pull request adds this intermediate method that builds the URLRequest from the URL and calls `setImageWithURLRequest:placeholderImage:success:failure:`

Also `setImageWithURL:placeholderImage:` was modified to just call this new method.