`mkdir -p` and `os.makedirs()` functionality. When true, ignore
FileExistsErrors. Patch by Berker Peksag.
(With minor cleanups, additional tests, doc tweaks, etc. by Barry)
Also:
* Remove some unused imports in test_pathlib.py reported by pyflakes.
Only possibly-controversial change: joinpath() was described as:
"Calling this method is equivalent to indexing the path with each of
the *other* arguments in turn."
'Indexing' is an odd word to use, because you can't subscript Path or
PurePath objects, so I changed it to "combining".