Mockfs

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* More Python 3 improvements.
* Switch to pytest.

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* Add Travis-CI testing.
* Only patch `os.getcwdu` on Python 2.
* Only patch `os.file` on Python 2.
* `setuptools` packaging.
* initial Python 2/3 support.
* Raise `OSError` when `os.makedirs` is used on an existing path.

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* Raise `IOError` when trying to write a file into a mockfs
directory that does not exist.
* Ensure that mockfs does close files that were never opened,
eg. when exceptions are thrown while saving files.

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* Support `open(path, 'rU')` for opening files.
* Source tree reorganization.
* Restored `install` and `uninstall` for the final v1.0 API.

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* Add `os.getcwd` and `os.getcwdu`.
* Enhance `os.rmdir` and `os.remove` to handle relative paths.
* Add `os.chdir` and `os.path.abspath`.
* Add `os.path.getsize`.
* Add `shutil.rmtree`.
* Add `os.unlink`
* Implement `open` and `close` for files by integrating Michael Foord's
storage.py.
* New `setup` and `teardown` replacements for `install` and `uinistall`
(REVERTED).

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* Add `glob.glob`.
* Add `os.makedirs`.
* Add `os.rmdir`.
* Add `os.remove`
* Raise `OSError` from `os.listdir` on non-existant paths.