Implement user authentication

[SilverBut]

Client-side certificate verify is originally supported by SSL, and I already have an implementation(untidy, though) for it. If we need to add it, I can just merge it.

Also, this will be the first authentication method added in the panel mentioned in issue #44

[NeatMonster]

Our best course of action to provide a reasonably secure-by-default approach is to allow using self-signed certificates. Custom PKIs could be used to verify the dedicated server's certificate, while an auto-generated certificate could be used for the integrated server if we perform some kind of trust-on-first-use.