attempt to approximate Google's latest branding guidelines

Fixes #15.

Note, your image file is scaled and sprited, but I don't think Git knows how to capture just the pieces of a PNG that change. This pull request includes a separate graphic for the new Google+ icon. It's up to you whether or not to incorporate this graphic into your sprite file.

Also, I shoved in a link to the Roboto font in the `index.html` file to further match the guidelines.