Add status

Add 102, 207, 208, 422, 423, 424, 426, 429, 431, 506, 507, 508, 510 and 511.

Asking a review from @hoaproject/hoackers :-).
You are right, 306 has been abandonned. Quoting [RFC7231]6.4.6. 306 (Unused)](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.

And in the official list, it is also referred as “unused”.