This is used to generate media query classnames for the demo content. The demo content boxes already have a `.pure-u-1` class name on them.

This helper looks at the number of items in the `mediaQueries` array, generates class names for each one, and appends it to the class list. On each iteration, it increases the denominator by 2, so it can get something like this:
```
pure-u-1 pure-u-{key1}-1-2 pure-u-{key2}-1-4 pure-u-{key3}-1-8
```
The default is 2, because thats where {key1} starts. By multiplying the denom by 2 each time, it's easy for the users to visually see the layout changing. 2 is also the lowest common multiple of 24.