Update OmeroPy/src/path.py

This puts path.py in sync with the [latest upstream version](https://pypi.python.org/pypi/path.py/5.2) (currently 5.2). The following is a recap of what happened to the changes introduced by the OMERO team:

* `read_md5` has been reinstated (i.e., upstream code has been left alone). According to the comments, it was removed due to changes in the hash libraries between Python 2.4 and 2.5, but now the minimum required version is 2.6; moreover, the method is currently not used anywhere
* Changes to `listdir` have also been squelched. The original behavior is consistent with that of `os.listdir`; the modified version, on the other hand, did not allow to distinguish between an empty dir and an unreadable one. [#9156](https://trac.openmicroscopy.org/ome/ticket/9156) is no longer an issue; `walk` calls broken by this have been fixed by adding `errors="ignore"` (note that, in this case, path.py is *not* consistent with `os`)
* the `parpath` method has been slightly updated to use the new convention for building a `path` object