Adds support for numpy number and dtype in _to/_fromdict

Supports numpy.number and numpy.dtype objects when running the _todict and _fromdict functions of Dictable
Merge pull request #78 from blazetopher/np_tofromdict

Adds support for numpy number and dtype in _to/_fromdict
Fixes bug with default fill_value
Addresses issue where default fill values are set as numpy.number objects rather than builtin types - can cause issues downstream