Any existing classes overwritten when switching tabs

If you choose to have classes on either the nav elements, or any of the targeted sections, they are overwritten when the active classes are applied.

This update uses a bit of code from the [Apollo.js project](https://github.com/toddmotto/apollo) to remove and add classes dynamically.

If you see any obvious reasons why this may not function properly in any of the supported browsers, please let me know!