Generate side margins of the displayed size

Uses `calc()` to display blocks of the correct size on the left and right of the content.

It avoids having two elements wide of thousands of pixels off-screen in memory and renders only the necessary size that is on screen.

Support of `calc()` is great on desktop (not so great in Android prior to 4.4: do we care?):

![screen shot 2014-07-24 at 12 07 15](https://cloud.githubusercontent.com/assets/85783/3687141/dc1e52a6-1322-11e4-8af9-cac3544de57f.PNG)

![ ](http://media1.giphy.com/media/mnFwhN0P2AVZS/giphy.gif)