Added HTTP codes as of RFC 6585

Added descriptions for the new HTTP codes:

- 428 Precondition Required
- 429 Too Many Requests
- 431 Request Header Fields Too Large
- 511 Network Authentication Required
Merge branch 'PHP-5.4' into PHP-5.5

* PHP-5.4:
  Added HTTP codes as of RFC 6585
Merge branch 'PHP-5.5'

* PHP-5.5:
  Added HTTP codes as of RFC 6585