Update index.php

Added block for the "Jump To" menu, displays in smaller handheld view with links to bento categories
Update index.php

Added anchor tags to bento category boxes
Update index.php

Adjusted visibility of the "jump to" block for 720px and lower.
Merge pull request #266 from gwu-libraries/t187-UI-mobile-jump-to

Adds jump-to links to anchor tags for single-column view