Refactored several methods to use ctypes exempi module.

Exempi library routines include xmp_new_empty, xmp_free,
xmp_get_property_float, xmp_get_property_bool, xmp_get_property_int32,
xmp_get_property_int64, xmp_get_array_item, xmp_set_property,
xmp_set_array_item, xmp_append_array_item, xmp_get_localized_text.  #issue2