add ip address based auth

[cyril-s]

Add simple whitelist IP address based authentication. List of allowed networks is passed using -allow.net flag. Useful if you have Linux router with lots of interfaces and lots of firewall rules to simply allow access only from prometheus.

[tcolgate]

I've generally avoided this in favour of TLS auth, but the implementation looks solid, and I'm less idealistic now that I got the TLS in where/when I cared about it :)

[tcolgate]

Thanks. It may be a while before I do a release and update the AUTHORS.