title: NSString: isEqual vs. isEqualToString
body: <p>What is the difference between <code>isEqual:</code> and <code>isEqualToString:</code>?</p>
  
  <p>Why are classes adding isEqualTo* methods (isEqualToArray for NSArray, isEqualToData for NSData, ...) instead of just overriding <code>isEqual:</code> ?</p>