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

errors.go

@@ -1,10 +1,18 @@
 package gowebdav
 
 import (
+	"errors"
 	"fmt"
 	"os"
 )
 
+// ErrAuthChanged must be returned from the Verify method as an error
+// to trigger a re-authentication / negotiation with a new authenticator.
+var ErrAuthChanged = errors.New("authentication failed, change algorithm")
+
+// ErrTooManyRedirects will be used as return error if a request exceeds 10 redirects.
+var ErrTooManyRedirects = errors.New("stopped after 10 redirects")
+
 // StatusError implements error and wraps
 // an erroneous status code.
 type StatusError struct {
@@ -32,7 +40,7 @@ func IsErrNotFound(err error) bool {
 	return IsErrCode(err, 404)
 }
 
-func newPathError(op string, path string, statusCode int) error {
+func NewPathError(op string, path string, statusCode int) error {
 	return &os.PathError{
 		Op:   op,
 		Path: path,
@@ -40,7 +48,7 @@ func newPathError(op string, path string, statusCode int) error {
 	}
 }
 
-func newPathErrorErr(op string, path string, err error) error {
+func NewPathErrorErr(op string, path string, err error) error {
 	return &os.PathError{
 		Op:   op,
 		Path: path,