Google Analytics

Tracks exercise visits as page views, and button clicks as events. Will, before release, count the number of attempts a user has made on a particular exercise.

This should provide very useful insight into the difficulty curve of the workshop and general usage patterns.

Analytics is enabled by default, but explained alongside the menu and convenient to opt out of.

![screenshot_2014-10-21_5 21 14 am](https://cloud.githubusercontent.com/assets/569817/4712728/7c289f32-58d2-11e4-9471-8e7430f768ef.png)