Send logs to a web socket with socket.io.

A log server can be started by specifying the `log_server_port` option. A `log_server_level` can also be set, else `log_level` will be used, else `2`. The web socket library used is [socket.io](http://socket.io), the event name is `log` and the message is an object with two properties: `level` and `message`. Example usage:

```
<script src="socket.io.js"></script>
<script>
  var socket = io('http://localhost:8089');
  socket.on('connect', function(){
    console.log('connected')
    socket.on('log', function(data){console.log(data)});
    socket.on('disconnect', function(){console.log('disconnected')});
  });
</script>
```

Once this change is merged, I'll need to modify [vagrant-solidus](https://github.com/solidusjs/vagrant-solidus) and [solidus-site-template](https://github.com/solidusjs/solidus-site-template) to automatically open a port for the log server in dev.