Add sponsorship to Series container

Actually a bit more complicated than I'd hoped, because the series component is ajax-ed in, after the adverts (might) have been displayed, so we need to make another call to dfp for the sponsorship logo

e.g.

![screen shot 2014-09-25 at 15 15 54](https://cloud.githubusercontent.com/assets/74132/4406079/6193404e-44c0-11e4-9b2c-4b076be8a028.png)

Agreed. Can update https://github.com/guardian/frontend/wiki/JavaScript-guidelines#variable-declaration to make it more obvious