Sort out content negotiation

[matt-wallis]

This is a catch-all issue - other more specific ones may be created too.
Reference doc: W3C: Best Practice Recipes for Publishing RDF Vocabularies/Content Negotiation.
We need to choose a recipe: essglobal uses RDF and Single HTML document (using PURLs) with a slash namespace, which means I think we follow recipe 4a.

The Best Practice doc refers to a content negotiation testing service but it suffered a DNS lookup failure when I tried it :-( Needs more investigation.

We need content negotiation for:

• RDF/XML
• SKOS
• HTML
• Turtle