Feature: allow configuring Socket.io to connect from different server

We wanted to continue to use the socket connection to send back the test results to the testem server from within our Django single page app running on a different port, so this change makes sure the socket connects on the right port.

i.e. pull in testem.js being served at localhost:9090/testem.js on a page served at localhost:8080, and it'll connect to the socket on localhost:9090.