Updating with OSX-specific instructions

Tested on OSX 10.9.5.
These instructions will soon be referenced from this tutorial page:  http://yeoman.io/codelab/setup.html.
The update for Mac users would be helpful as many are running into difficulties with permissions errors, and the tutorial page has been recommending using 'sudo' as a work-around.
The plan for the tutorial page is to recommend against using 'sudo' as the work-around, and instead point them to this page for the proper solution.