Path-py

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- Issue 44: _hash method would open files in text mode, producing
invalid results on Windows. Now files are opened in binary mode, producing
consistent results.
- Issue 47: Documentation is dramatically improved with Intersphinx links
to the Python os.path functions and documentation for all methods and
properties.

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- Issue 32: Add ``chdir`` and ``cd`` methods.

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- ``open()`` now passes all positional and keyword arguments through to the
underlying ``builtins.open`` call.

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- Native Python 2 and Python 3 support without using 2to3 during the build
process.

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- Added a ``chunks()`` method to a allow quick iteration over pieces of a
file at a given path.
- Issue 28: Fix missing argument to ``samefile``.
- Initializer no longer enforces `isinstance basestring` for the source
object. Now any object that supplies ``__unicode__`` can be used by a
``path`` (except None). Clients that depend on a ValueError being raised
for ``int`` and other non-string objects should trap these types
internally.
- Issue 30: ``chown`` no longer requires both uid and gid to be provided
and will not mutate the ownership if nothing is provided.

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- Issue 22: ``__enter__`` now returns self.