callback passed around to plugin

This enable a callback to be passed to ```eventTrack``` method, to fire logic after having sent the event.
This solves for exemple https://github.com/luisfarzati/angulartics/pull/167 where we wanted to wait for the event before redirecting.

```javascript
$angulartics.trackEvent('ui_click_link', { data: $scope.link }, function() {
  $window.location = $scope.link;
});
```

The callback needs to be called in the plugins so it will only work for custom plugins right now.