enable Content Negotiation for JSON-LD @context files

[elf-pavlik]

i started working on @context files for JSON-LD support
https://github.com/elf-pavlik/remotestorage-jsonld-contexts

we can either replace current html files with link to JSON-Schema or enable content negotiation so serve actual @context when requesting application/ld+json

one of existing URIs used as @context http://remotestorage.io/spec/modules/bookmarks/archive-bookmark/

[michielbdejong]

pasting

{
  "@context":
    "http://remotestorage.io/spec/modules/bookmarks/archive-bookmark",
  "id": "0a3f99e98cfc4e62a2df1d453b51bec9",
  "url": "http://jsunconf.eu",
  "title": "JSUnconf",
  "description": "JavaScript conference in Hamburg",
  "createdAt": "2014-04-13T12:19:38.969Z"
}

into http://json-ld.org/playground/ still fails, but now it is because of the lack of CORS headers on there