Added HTTP-based authentication

The purpose of this is in order to tie CalDAV accounts to an external web
application. This application must provide a URL end point to which a POST
request can be made, with the username/password sent as the payload. A 200 or
201 response is considered successful authentication. Any other response is
a failure.

radicale/config.py

@@ -51,6 +51,9 @@ INITIAL_CONFIG = {
         "stock": "utf-8"},
     "auth": {
         "type": "None",
+        "auth_url": "",
+        "user_param": "username",
+        "password_param": "password",
         "public_users": "public",
         "private_users": "private",
         "htpasswd_filename": "/etc/radicale/users",