Add support for WebDAV http methods

[0x7f]

I just wanted to do a small WebDAV test based on httpp. Found out that the methods defined in the WebDAV http extension are not supported by httpp atm, so I added them. Don't know whether you want them in the code base in the first place?

For details about the methods see: http://www.webdav.org/specs/rfc2518.html https://en.wikipedia.org/wiki/WebDAV

[0x7f]

Just had a look at node's http-parser and they do support even more methods. What do you think about adding all of them?

https://github.com/nodejs/http-parser/blob/feae95a3a69f111bc1897b9048d9acbc290992f9/http_parser.h#L93-L134

[daedric]

I'm totally fine to have all those methods supported :)