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.
Copy paste the license header from one of the other go files.