Add bottom margin to purchase form wrapper on single download pages

I think we should add a bottom margin to the purchase form wrapper on single download pages. 

Because there's no margin, anything underneath the wrapper now needs a margin-top to space it out

![1](https://f.cloud.github.com/assets/52581/882915/f5d7a8ce-f96e-11e2-9373-5ba829a18eac.png)

Because the ajax alert is positioned absolutely, it overlaps the text when item is added to cart

![2](https://f.cloud.github.com/assets/52581/882914/f5d10b9a-f96e-11e2-9ef5-97874e15105b.png)