This tool is a component of an auto-update system.  It is responsible for performing
the installation of an update after the necessary files have been downloaded
to a temporary directory.

This tool is responsible for:

 * Reading an XML file specifying the contents of an update.
 
 * Extracting and installing updated files from one or more compressed packages.
   
 * Removing any files which are no longer needed in the new version of the application.

 * Requesting elevated priviledges if required to install the update.

 * Displaying a simple updater UI and re-launching the main application
   once the update is installed.

The tool consists of a single small binary which only has a small number of external 
dependencies that need to be present on the target system.

The external dependencies of the updater binary are:

 * The C++ runtime library (Linux, Mac),
 * pthreads (Linux, Mac),
 * zlib (Linux, Mac)
 * native UI library (Win32 API on Windows, Cocoa on Mac, GTK on Linux if available)

To perform an update, the application (or another separate tool) needs to download
the updater binary, an XML file describing the update and one or more zip packages
containing the files for the update to a temporary directory.  It then needs
to invoke the updater, specifying the installation directory, temporary package
directory and path to the update script file.  The updater then installs the
update and restarts the application when done.