[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.

Within [assets/global/styl/variables.styl](https://github.com/Level/leveldb.org/blob/master/assets/styl/global/variables.styl#L28-L29), `$line-height-base` is `1.5` and `$line-height-tight` is `1.2000000000000002` after compilation from  [npm watch-styles](https://github.com/Level/leveldb.org/blob/master/package.json#L8). Note that in this pull request, `variables.styl` was never modified. I use the Contributing Guide's suggested [npm start](https://github.com/Level/leveldb.org/blame/master/README.md#L48), which does call `npm watch-styles`.