Add footer

Add a fixed footer with credit information at the bottom of the page.

I don't think we need to implement the expanding behavior as it
adds complexity without a lot of value.  The core information fits
nicely in the thin footer bar.  Other info can go on the about
page.

Fixes #75
Add favicon
Fix nav menus on narrow displays

Only use the circle icons on wide displays.  When the nav
collapses to a drop-down, show the nav items as text.

Only make the footer sticky on wider displays.
Move JavaScript out of index.html