title: iOS MKMapView zoom to show all markers
body: <p>I'm working with <code>MKMapView</code> and have plotted several points on the map. I have used the <code>MKCoordinateRegion</code> and <code>MKCoordinateSpan</code> to enable zooming etc around one of the points - but that's not what I want...</p>
  
  <p>I'm trying to use something similar to the Javascript zoom to bounds function. so all my points should be visible to the user. (There will be around 10 points around the UK) I'd like to show them all, or if most of them were in the London area, zoom to there. </p>
  
  <p>Is there a way to work this out programatically? </p>