Add :custom_vars option

Hello!

In the case of using `analytics_init ` method, if you add additional events by `add_events` option, it is set after that `_trackPageview` request.

However, for example, as is described in the [Google Analytics Documents](https://developers.google.com/analytics/devguides/collection/gajs/gaTrackingCustomVariables),  we need to set "_trackPageview()" request after setting a custom variable.
```
In certain cases this might not be possible, 
and you will need to set another _trackPageview() request after setting a custom variable.
```
(In my applications, _setCustomVar option has not been reflected in the Google Analytics.)

So, I changed the order in which additional events setting.
After change the order, _setCustomVar option has been reflected in the Google Analytics.

What do you think?

Thanks!