carray_ext.pxd

Hi Valentin,

as previously discussed per mail, here is the carray_ext.pxd pull request.
To check everything was fine I compiled, reinstalled and ran python bcolz/tests/all.py.

I tried to make as less changes as possible, but I still have some doubts.

Refactoring dtype parameter in _adapt_dtype function, was done only to be on the safe side, but
I am not completely sure if it is strictly necessary.

These cimport ndarray, dtype, npy_intp can still be found in both files (pyx and pxd)

Looking forward to any suggestions and improvements you have.

Regards,
Francesc