handle better HTTP authentication

[tc-gd]

I am not sure whether it is jenkins.el to be blamed or it is url package or it's just my configuration issue. Any help would be appreciated.

We're using jenkins service with kerberos authentication exposed via HTTP(s) authentication. Unfortunately when I use jenkins.el, I am repeatedly asked for credentials for that HTTP authentication.

Not only I am asked for password, but also username to be used.

[bhipple]

If we ported the HTTP request logic to use request.el, which has a curl back-end, we could fairly easily support Kerberos negotiation. This would also solve #18 and #29 by deferring the HTTP / authentication logic to requests.el and curl, leaving jenkins.el just handling the business logic.

It looks like we just have to port 3 function calls to do this. Any interest @rmuslimov? I might take a look at it eventually.