Add basic JSON operations as header only library

Add basic JSON operations as header only library

Usable JSON header-only library.
  - Applied design pattern: Composite pattern
    - Component structure: Value
    - Leaf structure: Int, String (To be added: Bool, Null)
    - Composite structure: Object (To be added: Array)

Usage:
    // Set json object
    Json json;
    json["name"] = "sangwan";
    json["age"] = 99;

    // Get json value
    std::string name = json["name"];
    int age = json["age"];

    // Serialize json value
    std::string serialized = json.serialize();

Signed-off-by: Sangwan Kwon <sangwan.kwon@samsung.com>