title: Natural logarithm method for NSNumber and NSDecimalNumber
body: <p>I've got some C# code that I'm converting to Objective-C. In C# I would call Math.Log(). I'm slowly learning that some people stick to C functions/types and only use NSNumber etc when they need to interop with Cocoa.</p>
  
  <p>Is there an equivalent for ObjC/Cocoa, or do I need to drop into C to do this? I need my code to be as accurate as possible because it is part of a calculator.</p>
  
  <p>This code needs to run on iPhones if that makes any difference.</p>