Use #[deriving(Clone)].
Use {En,De}codable for JSON output.

Makes the output less human-readable, but more Rust-readable: one can just load
the clean module and Decodable::decode should Just Work.

Unfortunately, there's no way to use Encodable to go straight to extra::json::Json
so this requires a trip via str (currently... hopefully this can be fixed).
Delete jsonify.
Merge pull request #28 from huonw/deriving

Deriving Clone, and {En,De}codable.