setup.py depends on numpy, pygrib is not installable from pip requirements file

[cpaulik]

This is a very common issue when installing several packages from a pip requirements file.

see https://github.com/scipy/scipy/pull/453

Should be solvable by using a solution like https://github.com/pydata/pandas/blob/master/setup.py#L92.

This introduces a setuptools dependency if not handled carefully.