Slightly more responsive ads, contextual styling.

Added several more sizes for ads, moved them to above the metadata instead of
above the whole content block.

Also added a layer of wrapping to the ad slot template which allows contextual
styling (e.g. an ad in this place should be decorated in **this** way).
remove banner which is no longer required
Merge pull request #84 from PRX/feat/ad_display

Slightly more responsive ads, contextual styling.