Decode JWT private claims

- Add a basic test around `jws.Decode`
- Decode private claims when decoding a claim set

Note that this puts all claims that are not set on `ClaimSet` into `PrivateClaims`. This is not strictly correct as there are a number of registered claims that are not used by this library. However, it seemed like the least confusing thing to do.
Remove the dot here, the purpose of `jws_test` is to allow usage of `jws` package qualifier in the sample.