OS filters / selector widget. Fixes DEVELOPER-193

Our installation guides are very consistent using DT an DD elements to give separate instructions for 

Rather than have the author wrap everything in a div, we can extract the information from the title having the words osx/mac/linux/windows in them.

![ss 2014-07-01 at 2 47 28 pm](https://cloud.githubusercontent.com/assets/176013/3447746/eec6320e-0150-11e4-8597-1fba4d539660.png)