Add status.

Add 102, 207, 208, 422, 423, 424, 426, 429, 431, 506, 507, 508, 510 and
511.
Update case of 418.
Fix messages for 308, 412 and 428.
Status 103 has never been standardized. Remove it.
Uncomment STATUS_IM_USED.

And update the RFC reference.
306 is now unused.

Regarding http://tools.ietf.org/html/rfc7231#section-6.4.6:

> The 306 status code was defined in a previous version of this
> specification, is no longer used, and the code is reserved.

Thanks @jubianchi for the review!
Fix message for 408.

Thanks @jubianchi for the review!
Add RFC references for all status.

Asked by @jubianchi.