Fix for #26: crash during rendering

This prevents `loadTileAtPath:` from making any calls to `result(..)` after the MBXMapView's has been dismissed, because that can cause a crash. The problem is mostly due to the completion handlers for download session tasks, since downloads may be in progress while the mapView is being deallocated.

@lightandshadow68, does this stuff:
```
[super setDelegate:nil];
_ownedDelegate.realDelegate = nil;
_ownedDelegate = nil;
```
correspond to what you mentioned at https://github.com/mapbox/mbxmapkit/issues/26#issuecomment-33705342 ? I didn't use MBXMapView's `setDelegate:` method because it includes a call to `reloadRenderer` which seems like it could easily cause further problems.