Adding read/write support for paths from pathlib

[frejanordsiek]

Adding read/write support for paths from pathlib (https://docs.python.org/3/library/pathlib.html). Since the package depends on this module now, it doesn't hurt to add the ability to marshal them. Unfortunately, only one of PosixPath and WindowsPath will be available on a particular system (and the other one not) and Path is essentially an alias for that one. This means that they cannot always be read back perfectly on a system different than the one that wrote them, but they can be converted to the next closest thing in such cases.