Add limitinfo in connection obj to keep latest API usage

Keep API usage information notified from API HTTP response header.
any reason why you are assigning these values after emitting response? would be nice if the values were set on the connection at the time this event is emitted.
I'm looking at doing something like: 

```
function getConnection(credentials) {
  var c = new jsforce.Connection(credentials);
  c.on('response', reportApiUsage);
  return c;

  function reportApiUsage() {
    // report c.limitInfo.apiUsage.used
    // report c.limitInfo.apiUsage.limit
  }
}
```

Setting the limit info values after emitting the `response` event means that I don't have current values in my `reportApiUsage` function.