doc for binding to unix domain socket

If host contains a `/` then it is assumed to be a path
to a unix domain socket and the server will bind to that
instead of using TCP

It will also set `server.info.unixDomainSocket` to the
absolute path to the socket.
add failing test for using unix domain socket
implement ability to listen on unix domain socket

If server option `host` contains a `/` then it is assumed
to be a unix domain socket and the server will listen on it
rather than TCP.

`server.info.unixDomainSocket` will contain the absolute path
to the socket it is listening on.
Merge pull request #1123 from jeffbski/unixDomainSocket

add ability to listen on unix domain socket