We were already doing this for bootstrap.properties-based launches, but there's
no reason to restrict it to that. If you run Getdown with an appbase and an
appdir, it can create the app dir and download the app into it.
This enables scenarios where getdown is installed to a shared location and
users that run the app install it in their private storage area (or on a
machine local file system, or who knows).
Closes#163.
Michael Bayne