Maps Pins do not display when PM Nav Bar is enabled

This is the weirdest issue I've seen with PM::MapScreen and how it interacts with the nav bar.

```
class Home < PM::Screen
title 'Map Screen'
nav_bar true

def on_load
map_view = MyMapScreen.new
rmq.add_subview(map_view.view).style do |st|
st.frame = {t: 100, w: device.width, h: 200}
end
end
end
```

In the MyMapScreen the annotations have been added and the pins setup correctly. However they don't display!

When I remove the nav_bar true, hey presto the pins appear!

I've tried all sorts of ways to fix this, manually adding a nav_bar:

```
UINavigationController.alloc.initWithRootViewController(self)
nav = self.navigationController.navigationBar
nav.frame = CGRectMake(0, 0, device.width, 70)
```

Or

`navigation = UINavigationBar.alloc.initWithFrame(CGRectMake(0, 0, device.width, 70))
`Anyway I've ended up going with the following and ditching PM Maps.

```
map_view = MKMapView.alloc.init
map_view.delegate = self
map_view.frame = CGRectMake(5, 80, device.width - 10, 200)
region = MKCoordinateRegionMake(CLLocationCoordinate2D.new(50.85, 4.35), MKCoordinateSpanMake(8, 8))
map_view.setRegion(region)
Beer::All.each { |beer| map_view.addAnnotation(beer) } #lifted from Samples
rmq.add_subview(map_view)
```

I'm not sure if anyone has had this issue, or similar, if so pls let me know, it would be nice to use the awesome map library.

Start at PM::MapScreen and the PM::Screen/Home example, reproducing the issue with nav_bar true and comparing it with the working direct MKMapView setup. Trace how annotations are added and displayed when the navigation bar is enabled; done means map pins render without requiring the reported workaround.

Written by the indexing model from the issue text.