add BatchGeocode to geocoding.go (+ test)
- remove json_result.go:
    - move common structs to geocoder.go
    - move geocoding specific structs to geocoding.go
- remove new as it was wrongly used in geocoding.go
- combine multiple fmt.Sprintf into one fmt.Sprintf statement
move defer resp.body.close after error checking so panic(err) could be called (https://groups.google.com/d/msg/golang-nuts/IgFmMqD65t4/zWHpOOS4ZOsJ)