Add option to prune empty directories

CVS has a -P option on the co command to prune empty directories.
After deleting files/directories in a project and committing those deletions,
we would like our CruiseControl/Maven builds (which use scm:update) to be able
to prune the empty directories so the CruiseControl checkout folder more closely
matches the developer's local files/directories. For example,
renaming a Java package (including subfolders) from a.b.c to a.b.d then committing to CVS,
leaves the old a/b/c and all subdirectories in the CruiseControl checkout folder after
scm:update finishes.