Once an update has been downloaded to a temporary directory,
this tool is responsible for installing the updates to the application's
directory and displaying a progress dialog whilst it does so.

Features:

 * Supports Windows, Mac, Linux

 * Standalone.  The only external dependencies are the C++ runtime library (Linux, Mac),
   pthreads (Linux, Mac) and the native UI library (Cocoa on Mac, Gtk on Linux)

 * A simple updater UI implemented using the native
   Win32, Cocoa and GTK libraries (depending on the platform).