Use np.intp_t for indices

Fixes many test failures of the following kind on win-amd64:

```
X:\Python27-x64\lib\site-packages\astropy\table\np_utils.py:233:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

>   ???
E   ValueError: Buffer dtype mismatch, expected 'DTYPE_t' but got 'long long'

_np_utils.pyx:19: ValueError
```