[renderjson.js] Add classes to all spans of text to enable syntax highlighting.

Classes available to colorize:
  .disclosure    (" ", " ")
  .syntax        (",", ":", "{", "}", "[", "]")
  .string        (includes quotes)
  .number
  .boolean
  .key           (object key)
  .keyword       ("null", "undefined")
  .object.syntax ("{", "}")
  .array.syntax  ("[", "]")
[renderjson.js] Whitespace
[renderjson.js] Comment
[test.html] Add empty array/object test.
[test.html] Colorize stuff using the new classes.
[README.md] Document the themable classes.
[renderjson.js] Add bits from the README into the code as comments, since nobody keeps READMEs around.