Bump minimum Numpy version to 1.4 - #70

I think this makes sense. I've had issues with 1.3 in the past for structured arrays, and these are going to be used in astropy.table.

The only issue I can see is that the last Ubuntu LTS release ships with 1.3. But the next LTS release is 12.04...which is probably sooner than we're likely to see significant Astropy adoption. So I don't see it as a likely issue...

What about having a setup.py flag to force installation regardless of dependencies? That way, python setup.py install would raise an Exception saying that e.g. Numpy is too old, but if you really want to install it, you can do e.g. python setup.py install --force.

I don't really care either way, but that would be easy to add if others think it's a good idea.

In any case I agree with not supporting 1.3 by default.

I vaugely prefer just requiring 1.4 because that'll be simpler/more-standard, but I'm fine with either solution.