Added delegate method for tracking when prompt gets shown

This delegate method is useful if you want to know when the prompt is shown. I use it for building Mixpanel funnels, so I can find out what percentage of people that get shown the prompt actually end up reviewing the app. From there, I can fine tune the settings for when/how the prompt gets shown to get the best chance at getting a good review. Maybe others will find it useful as well.

Thanks iRate, it's really useful!