Add helper classes for accessibility and invisible, focusable skip link.

This pull request adds some class names for accessibility usefulness and makes an invisible skip link that becomes visible when keyboard users tab to it. This helps make the skip link usable for not just screen reader users, but keyboard users as well.

**Unfocused**
![unfocused-skip-link](https://f.cloud.github.com/assets/1473618/1601909/283e9216-5390-11e3-82a9-bd954d14f51d.png)

**Focused**
![focused-skip-link](https://f.cloud.github.com/assets/1473618/1601915/389227f4-5390-11e3-9394-5eafbcf14c8c.png)