Added link to twitter account at the footer. Show tooltips instead of text for footer links.

Not yet implemented: email link. Make email address configurable.
Move upwards tooltips on footer links, so that they don't overlap the icons.
Added email link to footer. Moved tooltips so that all footer tooltips are vertically aligned.
Admin email is configured in application.rb. This email is then used in the mailto: link in the footer.

I'm hoping putting a mailto link in a highly visible place is not something I may regret later. Let's see if those antispam filters are doing their job properly.