Add a script to create a fake group

This fixes https://github.com/boxen/our-boxen/issues/235 in a more permanent way
Improving echo messages

Also changed to boxen standard of "No message if nothing changes"
Merge branch 'master' of https://github.com/boxen/our-boxen into fix_gid_issue
Changed to an ask for adding to /etc/groups

Bash y/n code adapted from http://stackoverflow.com/questions/226703/how-do-i-prompt-for-input-in-a-linux-shell-script
Speed up bootstrap by using checksums

We've been using this modified bootstrap here at GitHub for months. I've
neglected to extract it to the public repo, until now.

This uses a quick checksum of `Gemfile`, `Gemfile.lock`, binstubs, the
ruby version and bundler version to determine if bundling is
necessary. This is much, much faster than running bundler every time.

I've also included some of our other misc. changes, mainly to logging
output and some minor structural pieces.
Port over github's newer boxen auto-update code
Add linked docs about how automatic updates work
Merge pull request #665 from boxen/speedup-bootstrap

Speed up bootstrap by using checksums
Merge pull request #666 from boxen/improve-auto-update-ui

Improve auto-update UI
Merge branch 'master' into fix_gid_issue

Conflicts:
	script/bootstrap
Merge pull request #581 from petems/fix_gid_issue

Add a script to create a fake group
Merge remote-tracking branch 'upstream/master'