Unix domain socket support.

This patch adds Unix domain socket support to waitress, a feature I need for my work to ease deployment. This will fix #8 in the issue tracker.

I can't say I'm entirely happy with the patch. While I attempted to minimise the amount of code I had to touch, there are certain subtle differences in the behaviour between unix sockets and inet sockets (such as the behaviour of send() on closed sockets) that meant I ended up touching significantly more code than I'd intended.

However, I managed to make things so that the full functional test suite would run for both types of socket in full. The one place I skimped a little on test coverage was `cleanup_unix_socket()`, which isn't tested directly, and the case where the path points to a non-existent file is ignored. I'll fix that as soon a I get some rest.

I'd really appreciate it if you could review the patch and give any feedback you feel might be necessary.