Adds details for install on Mac OS X

@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.