Add button to skip to last seen frame

Store the last seen frame in a cookie, and add a button to easily jump
to that frame when revisiting the site.
Only display lastSeen button when data available

Avoid confusing people who don't have a cookie set with a button which
won't do anything.  Hide the lastSeen button and only display it if a
last seen frame is known.
Merge pull request #8 from aschrab/last_seen

Add button to jump to last seen frame