Horizontally center upload drop zone

Also adds a bit of spacing between the help text and the drop zone.

I saw this requested in some issue on the main repo, but I do not remember which it was.

Image showing how it looks with the code applied:

![screenshot 285](https://cloud.githubusercontent.com/assets/3382464/4379922/1a8fc23c-4365-11e4-8775-b5e9dc654c32.png)
Bootstrap has a `.text-center` class that could be used instead, but since CSS has no `@extend`feature and I did not want to add element style tags, I went with this approach.