Memory improvements

Application is using more memory than necessary when using the UIImageView-category setImageWithURL.

Two fixes:
On AFURLConnectionOperation: clean up NSOutputStream when ready (so after getting self.responseDate from this stream)
Clean up af_imageRequestOperation on UIImageView when ready (operation is no longer needed when image is loaded. This releases the operation and so releases the output stream).

Only the first fix is really needed, the second one is just cleanup.