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.
update XCode instructions re: boxen-web
OSX -> OS X
Update README.md

I found a superfluous space, and also because you are _in_ the directories you need to `./script/boxen` each time.