Declare Numpy as a setup dependency

This will install Numpy *before* matplotlib, so that we can get at its headers at compile-time.

Note that this doesn't work with `python setup.py install`, it only works when you do `pip install .` (I think that's by design).  But this should also fix `pip install matplotlib` from PyPI, which is awfully nice.

Thanks to @tcaswell, @WeatherGod and @pelson for their thoughts about this on the mailing list, and https://github.com/h5py/h5py/pull/356/files from which a lot of this is shamelessly stolen.