Add back to contents button.

This button conditionally shows when you are scrolled below the page contents and gives a quick way to get back to the page contents. I didn't go back go the root nav as it can be pretty large, and I think more people need an easy way to navigate around the current page.

![book-jump-link](https://cloud.githubusercontent.com/assets/24086/4785654/253b9d2c-5d83-11e4-9e30-952e66ec154a.gif)

Refs #1898