Added global cache PATH_CREATED used by 'mkpath()' to ensure it doesn't
try to create the same path more than once in a session (and, more
importantly, to ensure that it doesn't print "creating X" more than
once for each X per session!).
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.
that wrap them in the Command class).
Fixed 'copy_file()' to use '_copy_file_contents()', not 'copyfile()'
from shutil module -- no reference to shutil anymore.
Added "not copying" announcement in 'copy_file()'.
Wee comment fix.
