CRS objects are seriously not recommended.

Following up on discussion in #39.

I've removed them from the spec and only refer to them via refs to the original geosjon.org format spec. How can we say "not recommended" and yet show developers an ugly, terrible, complicated way to do it? (https://github.com/geojson/geojson-ld/issues/27 points to the right way)

Under interoperability considerations (expanded), I push all responsibility for use of non-default CRS onto servers and reiterate that servers must not under any circumstances give out lat,lng data. Serving lat,lng creates a world of hurt for developers (I hope we all agree).

To reiterate: this pull request makes CRS an application specific concern and points out that the CRS object from the original GeoJSON spec is one way (probably the most common way) to do it.

cc @geojson/owners @rajrsingh @dr-shorthair @jyutzler and also @elemoine (does this suit your needs, Éric?)