add optional-dependency on numpy

[warner]

Zooko and I agreed to add setuptools metadata to Tahoe, to declare a dependency on the "numpy" library if someone asks for the "reliability_tool" feature out of Tahoe.

I don't know if this will make it possible to auto-install numpy (I've never tried to easy_install it), but it *will* provide more information that we were offering before. It will also *not* require numpy for the general case, something which is important to me because the "reliability tool" web page is a tiny little piece of optional functionality and isn't worth the effort of compiling all of numpy. It's also important to me that we not burden users with the decision of whether they need numpy or not.