Short term hack to unbreak boxen-git-credential on 10.8 systems
Clarify where to create the new git repo.

Be more explicit about where the new repo should be located. It's much
easier to set up a repo on the internet. This was implied before and now
it's explicit.

The first time I helped a colleague set this up, we used a local repo.
Since we didn't know what we were doing, we screwed it up pretty badly.
This bit of text might allow others not to make the same mistake we did.
Clarify the installation steps after bootstrapping

Make it clearer on how to install a boxen from the command line after the
bootstrapping steps are done.
remove unused variable
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