BLD: add setup_requires in setup.py so numpy can be used with buildout

Numpy is a setup-time dependency due to some .h files, but is not declared as such. This causes tools like buildout ( http://buildout.org/ ) to fail.

Adding the ``setup_requires`` kwarg to the setuptools config shoud fix this: buildout will correctly assemble any declared setup-time dependencies, before executing the pandas build.

closes #3861