Added a strong License type. Includes tests.
Flip Json converters to never run unless a class expliictly opts-in.

Not that they would anyway, but this gives us extra protection in case
Json.Net somehow manages to spot them when deserialising other things.
ReleaseStatus is now a strong type.

- Includes tests.
- Includes support for converting from the old spec 'alpha', 'beta'
  types.
Merge pull request #389 from pjf/stronger_types

Use stronger types for module metadata.