Improve look of the json download link

Some improvements to the download link:

* Make left margin same as the rest of the page
* Add a top border which separates the download link from the rest of the page (the effect is more obvious when you view a section of code that doesn't have credits or history)
* Add a download icon (opting for a modified version of this [this icon](http://thenounproject.com/noun/download/#icon-No4501) (licensed CC0) rather than the typicons font)
* Make the download link not appear on printing