Expandable ad - DFP template 

This code takes the variables set in DFP and generates an "expandable" ad on the page.

The template code for DFP is here -> https://github.com/guardian/commercial-responsive-ads/blob/master/DFP__export-templates/Expandable%20V2.txt

Template in DFP is here -> https://www.google.com/dfp/59666047#delivery/CreateCreativeTemplate/creativeTemplateId=10029927

Because of the complexity of the template I have found it easier to use the import / export feature in DFP.

This gives us options to add up to 3 images (plus a background) for each half of the ad plus an overall background image. Each item can be positioned and repeated as needed. With this combination of options this should give us and advertisers enough options to create the ads.

Screengrab showing what happens when you use all of the image "layers" available.

![screen shot 2014-12-16 at 12 09 23](https://cloud.githubusercontent.com/assets/1303616/5453206/83ed816c-851c-11e4-9232-05861835050d.png)



don't think we should be using classes for their side-effects - all those classes could bring a load of extra baggage. can't we just put bottom margin on `creative--expandable`