Fix #148 leaving domain sockets open

In order to make sure the domain socket closes, we need to close() the
socket server itself, which requires paying attention to its creation
and

instruments/instruments.js

@@ -29,6 +29,7 @@ var Instruments = function(app, udid, bootstrap, template, sock, cb, exitCb) {
   this.eventRouter = {
     'cmd': this.commandHandler
   };
+  this.socketServer = null;
   if (typeof sock === "undefined") {
     sock = '/tmp/instruments_sock';
   }