Create custom analytics

We don't want to track our users via Google (at least not too much). Due to this we want to remove Google Analytics from hacken.in. We decided to go with [ahoy](http://ankane.github.io/ahoy/). This looks like a suitable solution for our needs.

 - [x] Setup ahoy to track page visits
 - [x] Use ahoy to track `IcalController` events (see: https://github.com/hacken-in/website/blob/create-nih-analytics/app/models/google_analytics_event.rb)
 - [x] Remove all usage of Google Analytics
 - [x] Enable time zone for MySQL (until we moved to Postgres)
 -  ~~Don't load fonts from Google~~
Don't we want those metrics as well?