Add typed getter methods to JsonObject

When a JSON object has optional members, code often needs to check if
the member was present before it can convert the value:

    JsonValue value = object.get("name");
    String name = value != null ? value.asString() : "unknown";

Allow to do this in a more convenient way:

    String name = object.getString("name", "unknown");

Yes, it's convenience API, however its potential to simplify user code
seems to justify the few additional methods.