Support overriding latlng-creation in GeoJSON.js.

Add support for overriding the latlng-creation with a custom factory method in GeoJSON.js.
Use case includes reprojecting the coordinates (for example by
reacting to the crs-field in the geojson spec).

This is one possible way of [Issue #866](https://github.com/CloudMade/Leaflet/issues/886). Really not sure if this is a good way or not though.