Changes suggested in #30.

No need to download http://www.opengis.net/def/crs/OGC/1.3/CRS84 is implied here. It's just a unique identifier.

OGR is one piece of software that currently writes "urn:ogc:def:crs:OGC::CRS84" into files. I'm not exactly sure what this change of wording means for OGR. Myself I have no problem with "http://www.opengis.net/def/crs/OGC/1.3/CRS84" and "urn:ogc:def:crs:OGC::CRS84" identifying the same CRS. Let's discuss.

@geojson/owners @sdrees @dr-shorthair