add xcode instructions for mavericks
update XCode instructions re: boxen-web
Merge pull request #485 from afeld/patch-1

Add XCode instructions for Mavericks
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.
Merge pull request #490 from jjasghar/patch-1

Update README.md