[scikit-learn] Declaring numpy and scipy dependencies?

[Andreas Mueller]

Hi all.
I think since the pipy ecosystem improved a lot, we should properly 
declare the scipy
and numpy dependencies, so that ``pip install scikit-learn`` works properly.
The argument why we did not do this previously was that this would try 
to download
and build numpy and scipy, which are pretty much guaranteed to fail / result
in a very slow installation. This is no longer true.

Any opinions?

Andy