Prefix all cl functions, add cl-lib dependency

The latter should make it work in Emacs 24.{1,2}, as long as this is installed
from a package repository.
Silence the "reference to free variable" warning
Don't rely on diff-mode faces changes provided by starter-kit
Depends -> Requires
Mention git-gutter
Use the line-prefix property

Intead of lots of tiny overlays
Highlight 'added and 'removed states, too