Valve has launched Steam remote downloads.
The feature, which was in beta, enables Steam users to manage their library of games remotely via the internet. If you leave Steam running on your PC or Mac, you can log into the Steam website or mobile access and download pre-loads, new releases and latest purchases, as soon as they're available. When you're home, your new downloads will be installed and ready to play.
You can do this from the Games list on the Steam Community site, which you can access from a web browser or the Steam mobile app. The Games list will reflect the installation state of your library on that machine. Find your game and hit install to trigger remote download.