Add Pathlib support in FileSet and FileInfo

[gerritholl]

It would be nice if Pathlib.Path was fully supported in FileSet and FileInfo. I haven't tested how (in)complete it currently works to add Path instances rather than strings representings paths, but at least no Pathlib is used at all in test_fileset.py, and although creating a FileInfo instances where path is a pathlib.Path instance, it currently (and with #378) compares unequal to another FileInfo instance where a string represents the same path, which I don't think is desirable.