MAINT: add install_requires to setup.py

[pv]

Tell pip and other tools that Numpy is required before build.

[rgommers]

-1, see http://article.gmane.org/gmane.comp.python.numeric.general/51758

[pv]

@rgommers: I thought there was some reason why it wasn't there, hence via PR... So the reason is that -U in easy_install and pip also causes upgrade of Numpy which can break stuff? Pip seems to have been fixed: pypa/pip#571 EDIT: or, actually not.

[rgommers]

Correct. And indeed, that pip PR is not in. sadly....