Adds details for install on Mac OS X

[willingc]

@audreyr, @riseriyo: Please give the Mac OS X install instructions a quick scan.

@alaindomissy: Since most mac users will be using OS X 10.6+, I think it is easier (less error prone) to send them to the main download page and click the yellow button that directly downloads the correct file.

OS X 10.5
I added install instructions for those with older macbooks and are still on OS X 10.5 (there are still a few that seem to still be). I put these in a separate cell so easier to delete or hide the cell if preferred.

A few helpful tips on mac
I also added a few tips for finding the OS version, finding files on a mac, and clarifying that $ is not typed when entering a command (you may be surprised the frequency of that question).

Attribution to other groups
I moved to the bottom of the doc since it looks a little prettier to my eye. Feel free to move it up if you desire.

[alaindomissy]

@willingc looks much better
I will shortly make a similar change for the windows instructions:
give this link https://www.python.org/downloads/
on that page the big yellow button will download the appropriate installer for windows