Removing crsURN

This makes all GeoJSON positions latitude, longitude [, elevation in meters] relative to an ellispoidal CRS based on the WGS84 datum.

I'm in favor of restricting the allowed coordinate reference systems for GeoJSON objects to 1: CRS84.

I think the second best alternative would be to restrict to 2: CRS84 or EPGS:3857. But that means we'll only be cool as long as Google Mercator is cool. And 2 is pretty arbitrary. Soon people will want 3.

I don't like the "allow any CRS and let axis order follow the CRS" because I think it either reduces interoperability or imposes an unreasonable burden on web clients (I don't know of a good web service - or really want to depend on one - that provides axis order information for arbitrary CRS URN, and the table is too big to ask every client to carry around).

With all GeoJSON coming in a known CRS, clients that want to consume any GeoJSON but work or render in a different CRS shouldn't have a hard time finding code to do coordinate transforms. This is a smaller burden than being able to look up axis order for any CRS URN.

(Note that this is a second pass at #2.)