use buffer interface instead of NumPy to export Julia arrays

This would eliminate the NumPy dependency (and the ugly parsing of NumPy header files). See [PEP 3118](http://www.python.org/dev/peps/pep-3118/#the-py-buffer-struct) and the [Python buffer docs](http://docs.python.org/2/c-api/buffer.html).

cc: @jakevdp, @StefanKarpinski