[Yt-dev] Upgraded installation script

[Matthew Turk]

Hi all,

Heads up: I upgraded the installation script to mercurial 1.7 and
numpy 1.5.1.  I've also added Cython (0.13) to the installation
script.

As a note, I'm holding off updating Python to 2.7 simply because
(while I can't speak for anyone else) I have some serious muscle
memory for "python2.6" which will take a while to convert to
"python2.7".  But there are some cool things we'll eventually be able
to use in Python 2.7, even if we never did use the "with" statement in
Python 2.6.  :)

-Matt