DOC: comprehensive rewrite for OSX binary install

Katy's recent edits were a big improvement, but the faq install instructions
for OSX had also got horribly out of date.  The main thing that has changed is
that wheels are now the default binary installation format, and, even if we are
going to provide eggs, no-one should be using easy_install now.
typo: "Once" -> "One" or "A"