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.