Feat: Authentication API

The changes simplify the `req` method by moving the
authentication-related code into the API.
This makes it easy to add additional authentication methods.

The API introduces an `Authorizer` that acts as an
authenticator factory. The authentication flow itself
is divided down into `Authorize` and `Verify` steps in order
to encapsulate and control complex authentication challenges.

The default `NewAutoAuth` negotiates the algorithms.
Under the hood, it creates an authenticator shim per request,
which delegates the authentication flow to our authenticators.

The `NewEmptyAuth` and `NewPreemptiveAuth` authorizers
allow you to have more control over algorithms and resources.

The API also allows interception of the redirect mechanism by setting
the `XInhibitRedirect` header.

This closes: #15 #24 #38

cmd/gowebdav/main.go

@@ -55,6 +55,10 @@ func main() {
 
 	c := d.NewClient(*root, *user, *password)
 
+	if e := c.Connect(); e != nil {
+		panic(e)
+	}
+
 	cmd := getCmd(*method)
 
 	if e := cmd(c, flag.Arg(0), flag.Arg(1)); e != nil {