Download and install numpy from source if not found

This is a port of a magic incantation recently installed into matplotlib, and orginally from h5py.

If Numpy is not installed, setuptools will download and build it, then use that to build astropy.  This has worked with `pip install astropy` for some time, but this now works with a source checkout as well.

The question of whether to upgrade the minimum numpy 1.5 is left as a separate issue.