Add HTTP Auth to webhook notifications#377

When Uptimerobot is fetching Callback URLS (Webhooks) that we design to act upon a downtime is detected (log, restart service, etc), we currently have to name these URLs with very obscure and long names to make sure (hope) that hack robots do not find it. It should be VERY suitable that we can provide credentials to Uptime Robot in order to allow us to protect theses pages by a 401 challenge.

UptimeRobot calls Callback URL the simplet way possible.
If we, webmasters, protect these URLs with 401-HTTP Challenge, aka the most basic, efficient and historical way of protection, theses Callback URLS cannot be accessed.

UptimeRobot must provide us a way to provide login + password credentials, in addition to the Callback URL.