Doesn't NSLog SecItemCopyMatching failed if not found in objectForKey

SecItemCopyMatching can return the item not found code. In the objectForKey method, having it NSLog "failed" if an item is not found can lead to endless log statements if objectForKey is used repeatedly.
Merge pull request #17 from NatWeiss/patch-1

Do not log SecItemCopyMatching failure if key not found in objectForKey