Toolrack

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- Add ``TempDirFixture.join``.

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- Finalize switch to python3.
- Code cleanups.

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- Switch to python3.

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- Add description to ``ConfigKeys``.
- ``Config.keys()`` now return a list of sorted ``ConfigKeys``.
- Fix issue with ``flatten_dict()`` when the key is not a string.
- Add ``Collecion.sorted()``.

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- Added ``toolrack.config module``.
- Fixed use of ``mkstemp`` in ``TempDirFixture``.

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- Added modules:
- ``toolrack.collect``
- ``toolrack.convert``
- ``toolrack.iterate``