Auth with password

[jamesread]

This might be useful for very simple, quick and easy setups.

HTTP Basic auth could be considered if it doesn't add too much code bloat to the HTTP Servers.

I'm not interested in implementing per-user passwords, as we'll quickly get into a situation of having to implement a full user management stuff. I'm thinking a simple config option like AuthSimpleAdminPassword.

As suggested by @bmitchinson in #30 (and some others) in other places.