[matplotlib-devel] site docs

For the 0.91 release, I have updated the web site, which had become
woefully out of date.  In addition to some 91 specific things like
"what's new", credits and updating a bunch of links to point to
pyplot, I also cleaned out some dead wood (removed numeric and
numarray references, cleaned up installing and backends to an extent).
 I think a lot more needs to be done to get the site fully modernized
(eg installing could use a lot more work, goals needs to be
modernized), but if you see something egregious or otherwise in need
of attention, let me know, because on these kinds of things it is good
to strike when the iron is hot.

Also, it has been a long time since I updated the credits page, so
this too is very stale.  If you are not mentioned there and would like
to be, or if I have left out some significant contributions that you
would like to see there, please drop me a blurb and I'll update it.

JDH