Fix for issue #9 (simplestyle)

This adds simplestyle support for issue #9 by fetching simplestyle geoJSON, and for maps which have markers, adding the markers to the map using MKPinAnnotationView (i.e. generic pins).

Noteworthy things:
1. This incorporates the lessons learned from #23 about network error handling (see `updateOverlay` and `updateMarkers`)
2. The example map is from weekend picks (see #44) which includes markers but no lines or polygons. This pull request wasn't attempting to implement any support for lines and polygons though, so that's fine.
3. There is some related background info at #21 (prior attempt at a simplestyle implementation)