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.
Merge pull request #478 from mattr-/clarify-install

Clarify the installation steps after bootstrapping
remove unused variable
Merge pull request #481 from grosser/grosser/fetch

remove unused variable
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.
Merge remote-tracking branch 'upstream/master' into spring-cleaning