Support nullable types, including primitives, enums and structs. Tests included.
Nullable type support based on https://github.com/chipdude/litjson/commit/eaa84eed6e0d876b962c5b33b4fc4a01b4fdbd3b
Getting pull request #6 ready

- Added NEWS item
- Code style fixups and fixed warning in JsonMapperTest
Merge branch 'michaelbartnett-support_nullable_types'