Add Google Plus as a comments widget in classic theme

Adds the ability to use Google+ for comments. I know a lot of the way this works is probably changing in 3.0, but I'm not sure what the state of Octopress development is right now so I figured I'd start pushing back some of the changes I wrote for my own site, then updating them to work in 3.0 if necessary.

A preview can be found (temporarily) on my blog. Eg. https://blog.samwhited.com/2013/06/new-ssl-certificates/
I may or may not retain the feature, so if it's gone just ask and I'll put it back. This probably won't work in preview-mode because Google will try to check the window location to load comments.