JSON: add support for "null"

[kud1ing]

"json.rs" currently has no support for the JSON value null (http://en.wikipedia.org/wiki/JSON)

Please

• add support for the JSON value "null" to libstd/json.rs
  • encoding: either add "Null" to "tag json" or "make to_str()" accept an "option::t".
  • decoding: should be fine, because "from_str_helper()" returns "(none, s)" for anything it does not know about
• add a test-case for the JSON value "null" to test/stdtest/json.rs, where you cover
  • encoding
  • decoding