Support binding UNIX sockets

[spladug]

I've an application that I'd like to run under Einhorn. It currently binds a UNIX socket and then an SSL-terminating nginx sitting on the same box talks to the service over that socket. This is a websocket service so it's designed to have tonnes of open sockets and using UNIX domain sockets for the nginx<->service bridge gives us a lot more headroom. Would it be possible for Einhorn to be able to bind UNIX sockets as well?

I'm happy to write the code for this (have taken a short crack at it already) but I'm a) not great at ruby, and b) would probably need some handholding on making sure the in-place upgrades are OK with the changes. If I do write it would this be something you'd want to merge?