json: add sub-section on default de/enconding (staging area)

Cover deconding from &str, default decoding and encoding into String (ref. #86)

(Similar to the previous PR, with added encoding)
Do you usually use `MemWriter`s when dealing with json encoding? I have never used `MemWriter`s.

For encoding, I usually do: `[deriving(Encodable)]` + `json::Encoder::str_encode`. I doesn't produce a "pretty" format, but on the bright side it doesn't generate `Err`ors, so I don't have to unwrap.