Use interface{} (or map[string]interface{}) instead of string for json content

[didiladi]

@patrickpichler gave an awesome feedback here:

Why do we unmarshal the json as a string?
This seems a bit odd to me. Wouldn't something likeinterface{} be better?

Describe the solution you'd like
Replace the json string with interface{} or map[string]interface{}. This makes the extraction of elements easier, and its more efficient. This makes the following possible:

var result map[string]interface{}
json.Unmarshal([]byte(empJson), &result)

stuff := result["stuff"].(map[string]interface{})
foo := stuff["foo"]