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 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):

    https://support.google.com/plus/answer/1713826?hl=en

However, that seems to interfere with the rendering of actual A tags in
the BODY, so I think we should try using a LINK tag and see if Google
will pick that up.
Merge pull request #2018 from gittip/plus-google

Add a link to our Google+ page