DAR-1428: Add responsive grid to responsive layout.

https://wikia-inc.atlassian.net/browse/DAR-1388

Here is the responsive grid that was prototyped earlier today. I haven't had time to test it in Internet Explorer yet, but it should work fine in versions 9 and up (looks great in Safari, Chrome and Firefox).

I wasn't sure where the best place to put a demo page was, so I just stuck it in the `/tests` folder for now. Feel free to move it somewhere else (bonus points for getting it to work directly with the Sass file).

Note: This grid will eventually replace the current "WikiaGrid" which uses hardcoded pixel values.