Commercial: add Media Math tags

Basically a port of https://github.com/guardian/r2-frontend/blob/master/r2frontend-static/src/main/resources/common/scripts/mediamath-tag.js 

![1285842896_sidewalk-car-crash](https://cloud.githubusercontent.com/assets/74132/2792537/322111e8-cbd4-11e3-9223-8ac607f0e836.gif)