DOC: comprehensive rewrite of OSX install docs

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instructions for OSX had also got horribly out of date.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.
DOC: extend instructions for Python.org Python

Also - point to 'getting help' section in troubleshooting.
DOC: fixes suggested by Thomas

Remove debugging pip for Python.org Python.

De-escalate problem heading.
DOC: fix cruft from bad edit of windows section

Thanks to Eric F for pointing it out.
DOC: fix once -> one typo; link on system python

Fix typo pointed out by Eric F; add link to wiki page on system Python
and Extras/lib.
DOC: make clear role of OSX version in dmg name

OSX version in dmg name in fact refers to Python.org Python
compatibility - but not the version of OSX it was 'built for'.
DOC: suggest xcode then help instead of vice versa

From suggestion of Eric F.