Xt

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- Added missing docstrings (80).

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- Support a single zoom value passed to ``tiles()`` as advertised (78).

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- The ``xy`` function returns ``float(inf)`` and ``float(-inf)`` for y at the
North and South pole, respectively, instead of raising an exception (76).

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- Corrected an error in the ``bbox`` parameter description in the
``bounding_tile`` docstring (73).
- Corrected an error in the geojson.io example in the CLI docs: the proper
usage is ``mercantile shapes --collect`` (71, 72).
- Add missing ``--version`` option to mercantile command.
- Python 3.6 has been added to the Travis build matrix.

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- Add ``tiles`` to ``__all__`` and sort that list. This sorts the classes
and functions in the API docs.