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 requesting weights 400, 500, 700 and 400italic.

Is there a reason to requesting different weights ? E.g. 700 is the default "bold" weight, so I think we might want that. And some italic would be handy as well :wink: