Version 0.18.0

   +  Fixed crash when ``modelpath`` is blank and ``task`` is not
      ``cross_validate``.
   +  Fixed crash with ``convert_examples`` when given a generator.
   +  Refactored ``skll.data``'s private ``_*_dict_iter`` functions to be
      classes to reduce code duplication.

@mheilman, do these changes break henry?