Rubygem level gittip integration (WIP)

This is the work necessary to make a gittip icon appear on each rubygem page where any of the users has opted into gittip on rubygems.org.  It may or may not be entirely complete, but I've got to stop work for a moment, so I'm getting the pull request up in case someone else can finish it off before I can.

The related PR on gittip is zetaweb/www.gittip.com#575.