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.
These where the instructions to use @import from [Google Fonts](http://www.google.com/fonts). We could use `<link>` like you suggested below.