Remove non-working git commands from quickstart.

You can't check out subdirectories with git, at least without doing something fancier.
DOC: Improve wording of a small part of the quickstart
Fix examples path for OS X.

It's probably broken for the other OSes as well, but I can't check it.
Merge pull request #88 from bgrant/improve-quickstart

DOC: Improve the Quickstart document