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)
Sounds good to me. If you are going to use an abbreviation for longitude and
latitude, maybe you can add a comment with the expanded form to the struct
fields.

``` rust
struct Location {
    // Latitude
    lat: f64,
    ..
}
```