Emit the `connection` event before the `data` event

When using `setImmediate`, there are cases where the `data` event on the spark is called before the `connection` event on the primus instance:

```js
primus.on('connection', function (spark) {
  console.log(spark.id + ' connected');
  spark.on('data', function (data) {
    console.log(data);
  });
});
  
for (var i = 0; i < 10; i++) {
  (new primus.Socket(url)).write('hello ' + i);
}
```
This causes the loss of some messages because we add the listener for `data` after the `connection`, so i think it's better to only use `process.nextTick`.

**The integration test included with the patch does not pass with the `browserchannel` transformer. All the connection are created but the message is received only for the first four. This needs investigation.**

**Update**: the test wasn't passing because by default the maximum concurrent sockets that the http agent can have open per origin are 5.
This number has been increased to 15 using `http.globalAgent.maxSockets = 15;`.