MapScreen: check for annotation specificities

Hello,

I was using `MapScreen` without too much issues, but when I tried to show my user's location (using `mapview.showsUserLocation = true`), I got errors of the kind:

    Terminating app due to uncaught exception 'NoMethodError', reason: 'map_screen_module.rb:94:in `mapView:viewForAnnotation:': undefined method `annotation_params' for #<MKUserLocation:0xa44eea0 (NoMethodError)

What I think happens is that iOS uses a custom annotation class, and the current code takes for granted that `annotation` responds to `annotation_params`, which is not the case in that situation.