docs(app): adds roboto font

The Google Material design docs at http://www.google.com/design/spec/style/typography.html state that we should use the font 'Roboto' for our Typography. I went ahead added the font using @import, but we could also download the fonts and add them to the project. I'm fine with either way.
On the [Material Design Spec](//www.google.com/design/spec/material-design/) they are using `sans-serif` as a fallback, so I think it would be better to use that as well:

    body {
        font-family: Roboto, sans-serif;