refactor(Recipe): rename constructor fromJsonMap to fromJson

- Follow the convention of symmetrically naming to/from JSON converters
as illustrated in [Dart Up and Running, Chapter 2 section on
"Classes"](https://www.dartlang.org/docs/dart-up-and-running/contents/ch
02.html#classes) — viz. showing named constructor `Point.fromJson()`.
- Also using literal constants for new Map and new List.
Reverting to new X<>() notation rather than [] or {}
Merge pull request #99 from chalin/patch-0312-recipe-json-ctr-rename

refactor(Recipe): rename constructor fromJsonMap to fromJson