Provide a file download URL for desktop app

[zbentley]

This is pretty trivial.

brew cask and other package managers prefer (but do not insist on) a "real" file-download URL, which is to say that they like a URL that looks like a file path and ends in .dmg. This preference is because it is often easier to deliniate between e.g. beta versions, different OS versions, etc. of the same package.

Currently, the DMG download is available from a non-path-ish URL: http://download.classeur.io/download/osx.

Could a .dmg URL be added to the landing site? Totally fine if not. Again: this is so low-impact as to be nearly cosmetic.