NSString method to truncate to required width using a given font

I've used this method a couple of times, first with [John Muchow's implementation](http://iosdevelopertips.com/cocoa/truncate-an-nsstring-and-append-an-ellipsis-respecting-the-font-size.html) and then with another one based on [a StackOverflow question](http://stackoverflow.com/q/10693383/592454) (which evolved from John Muchow's as well)