Added WebDAV status codes

There are WebDAV methods exists but status codes are not. Added codes from http://www.webdav.org/specs/rfc4918.html#status.code.extensions.to.http11
Merge pull request #574 from TrueBers/patch-1

Added WebDAV status codes
Merge branch 'master' of https://github.com/rejectedsoftware/vibe.d