[ux] Adjust styling of large buttons for smartphone res.

![before after](https://cloud.githubusercontent.com/assets/2647488/3183675/d0e79ba8-ec72-11e3-907b-56dd48814d43.png)

This pull request targets the styling for smartphone resolutions' (max-width: 480px) by adjusting the margins accordingly to center all large buttons and gives the `C++` and `Node.js` bindings buttons spacing between each other.

The change reflected here eliminates the extra trailing whitespace after and below `.intro`.