This PR aims at implementing compliance to RFC7519, as documented in #11 without breaking the public API. It creates a new struct `RegisteredClaims` and deprecates (but not removes) the `StandardClaims`. It introduces a new type `NumericDate`, which represents a JSON numeric date value as specified in the RFC. This allows us to handle float as well as int-based time fields in `aud`, `exp` and `nbf`. Additionally, it introduces the type `StringArray`, which is basically a wrapper around `[]string` to deal with the oddities of the JWT `aud` field.
John Maguire