Path-py

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- Issue 20: `relpath` now supports a "start" parameter to match the
signature of `os.path.relpath`.

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- Minimum Python version is now 2.5.

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- Issue 5: Implemented `path.tempdir`, which returns a path object which is
a temporary directory and context manager for cleaning up the directory.
- Issue 12: One can now construct path objects from a list of strings by
simply using path.joinpath. For example::

path.joinpath('a', 'b', 'c') or
path.joinpath(*path_elements)

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- Issue 7: Add the ability to do chaining of operations that formerly only
returned None.
- Issue 4: Raise a TypeError when constructed from None.