Basic authentication

[narc0tiq]

I've written a little bit about how to do authentication at http://wiki.narc.ro/a/Guns_Auth_indev -- basically, we use RSA keys to encode/decode a few bits of data in order to verify that client and server are who they say they are.

I've also implemented part of this, starting around commitish bd695f0. Naturally, with no metaserver yet and only a tenuous understanding of what the final implementation will look like, we're not putting this into the master branch yet, but it will have its own branch where we can screw around with it.