Implicitly required

Currently based on #17, two small changes:
  - 8391f95: make non-nullable valuetypes implicitly required in model description. This should work in both  DSL as annotations strategy.
  - 08718af: Alphabeticaly order resources, apis, operations, models and modelproperties to make output deterministic. This is more convenient in the acceptance tests (and it also looks nicer ;-))