'run_command()' to refer to it before attempting to run a command --
that way, command classes can freely invoke other commands without fear
of duplicate execution.
Beefed up some comments and docstrings.
meaningful return values: respectively, whether the copy was done, and
the list of files that were copied. This meant some trivial changes in
core.py as well: the Command methods that mirror 'copy_file()' and
'copy_tree()' have to pass on their return values.
and dry-run flags consistently painless): 'execute()', 'mkpath()',
'copy_file()', 'copy_tree()', 'make_file()', and stub for 'make_files()'
(not sure yet if it's useful).
Greg Ward