adding http status codes according to rfc6585

While building an API webservice, we wanted to use HTTP Status `429`, as proposed in [rfc6585](http://tools.ietf.org/html/rfc6585):

     The HTTP Status Codes registry has been updated with the following entries:

      Value: 428
      Description: Precondition Required
      Reference: [RFC6585]

      Value: 429
      Description: Too Many Requests
      Reference: [RFC6585]

      Value: 431
      Description: Request Header Fields Too Large
      Reference: [RFC6585]

      Value: 511
      Description: Network Authentication Required
      Reference: [RFC6585]

It would be great, if could have these built-in. Funny though, `428` was already present.