fix: style.css media-query for mobile devices

Current `@media only screen and (min-device-width: 810px)` prevents mobile devices to load major styles, hence the book contents are nearly unformatted (refer to the screenshot from iPad below). This commit removes the `min-device-width` condition.

![photo dec 28 11 44 50 am](https://cloud.githubusercontent.com/assets/80781/5563064/bc92d37a-8e8e-11e4-8124-f198bd293579.png)