title: MapKit uber style map Center annotation
body: <p>I would like to have a annotation in the center of the map, that does not move when the region is changed. That is, when one moves the map, the annotation stays in the center regardless. I have implemented this using NStimer, assigning the map center coordinate to the annotation every .001 seconds, however this is very choppy and doesn't look smooth.</p>
  
  <p>I realize that I need to use an Overlay than convert the overlay annotation to coordinates on the map. </p>
  
  <p>Pretty much it would look like what Uber has. </p>