callback passed around to plugin

This enable a callbacks to be passed to ```eventTrack``` method.
This solves: https://github.com/luisfarzati/angulartics/pull/167 in a cleaner way.

Registering the callback:

```javascript
$analytics.eventTrack('ui_view_click_google' , null, function() {
  $windows.location = 'http://www.google.com';
});
```

Handling the callback in a custom plugin:

```javascript
(function(angular) {
  'use strict';

  var module = angular.module('app.angulartics', [
    'angulartics'
  ]);

  module.config(function($analyticsProvider, $api) {

    var send = function(name, data, callback) {
      $api.events.create({
        name: name
        data: data
      }).then(callback)
    };

    $analyticsProvider.registerEventTrack(send);

  })

})(angular);
```