Secure p2p session connection

[nlloyd]

Right now completely unsecured, basic connection. Should fix that at some point.

[glyph]

I should note that if you use Vertex – as mentioned on #13 – you will almost get this for free. It doesn't currently do this, but since Twisted moved to using a memory BIO (i.e. the introduction of twisted.protocols.tls) you could run its TCP-over-UDP channel over regular TLS, and it has all the certificates you need for the connection to do it.

I filed the bug in Vertex here: https://bugs.launchpad.net/divmod.org/+bug/1099041 in case you want to work on it. I'm sure anyone on the divmod-dev list would be happy to help you with that contribution. :)