Utm

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* fixed missing zone letter for latitude 84 deg.
* fixed ``from_lat_lon()`` longitude error message
* fixed zone numbers for 32V and related regions

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* added optional ``force_zone_number`` parameter to ``from_latlon()`` (`8 <https://github.com/Turbo87/utm/pull/8>`_)
* fixed minor precision error (`9 <https://github.com/Turbo87/utm/pull/9>`_)

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* added optional ``northern`` parameter to ``to_latlon()``
* use `py.test <http://pytest.org/latest/>`_ instead of `nosetest`

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* return floats from ``from_latlon()``

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* more unit tests

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* performance improvements