Add a link to our Google+ page

From their instructions:

> By adding a short line of code to https://www.gittip.com/, you can make your Google+ page eligible to show up on the right hand side of the Google search page for relevant queries and make your Google+ page more discoverable.

[These instructions](https://support.google.com/plus/answer/1713826?hl=en) indicate that we can include a text-less A tag in the HEAD rather than an actual link in the page (which would require a little more thought).