It would be nice to make it so that the "dist" commands and "install" would
automatically do darcsver before proceeding.  This would avoid trouble with
having a stale version number and forgetting to update it.  How can they be
configured to do this, though?  Wouldn't it be nice if setuptools accepted a
callable optionally instead of a string for "version" and invoked that callable
whenever it needed to know the version number?