Allow coordinates with more than two elements

Thanks for this library.

I have a [data set](http://daten.berlin.de/datensaetze/geometrien-der-ortsteile-von-berlin-stand-072012) with coordinates that have three elements:

````
                "coordinates": [
                    [
                        [
                            13.37362286299343,
                            52.52791796547066,
                            0
                        ],
````

When reading this data set with `shapesFromGeoJSONFeatureCollection:error:`, the current code  causes an assert because `CLLocationCoordinateFromCoordinates()` expects exactly two coordinates.

The GeoJSON spec allows for more than two elements, see [2.1.1. Positions](http://geojson.org/geojson-spec.html#positions). For this reason, I modified the code to ignore additional elements.

Please have a look, thanks.