feat(header): Support arbitary status codes

As discussed in #177 hyper must support status code outside the standard range for compatibility reasons.

BREAKING CHANGE: This removes unregistered status codes from the enum. Use FromPrimitive methods to create them now.
StatusCode and StatusClass can no longer be casted to u16, use ToPrimitive methods now.
For example `status.to_u16().unwrap()` to get the status code number.