title: provide simple method to get current speed (implementing speedometer)
body: <p>Could you please give me example how to calculate current "speed", I'm working on my first simple app ala speedometer?</p>
  
  <p>I figure out to use <code>didUpdateToLocation</code></p>
  
  <p>also I found that I need to use formula <code>speed = distance / duration</code></p>
  
  <p>Is it right? how to calculate duration?</p>