allow inspection of the data used by PyArray_EquivTypes

Changes include:

 - Added the PyDataType_HASSUBARRAY macro
 - Added the new NPY_DATETIME and NPY_TIMEDELTA entries to NPY_TYPES
 - Added the "kind" and "subarray" fields to the dtype class

Additional work could be done to expose the datatime metadata, but
non-performance users can already get at that data using the pure
python numpy.datetime_data function.

Author: Gerald Dalley