add xcode instructions for mavericks
Update script/bootstrap sudo to pass environment variables.

When executing a boxen run from inside the corporate firewall we need to pass
proxies through to sudo so that bundler will be installed properly as root.
Merge pull request #489 from johnbellone/patch-2

Update script/bootstrap sudo to pass environment variables.
update XCode instructions re: boxen-web
Merge pull request #485 from afeld/patch-1

Add XCode instructions for Mavericks
OSX -> OS X