git-blame: Change installation instructions

git-blame: Change installation instructions

Change installation instructions to using either "(require 'git-blame)"
or appropriate autoload instruction in GNU Emacs init file, .emacs
This required adding "(provide 'git-blame)" at the end of git-blame.el
and adding [preliminary] docstring to `git-blame-mode' function for
consistency (to mark function as interactive in `autoload' we have to
provide docstring as DOCSTRING is third arg, and INTERACTIVE fourth,
and both are optional).  `git-blame-mode' is marked to autoload.

While at it ensure that we add `git-blame-mode' to `minor-mode-alist'
only once (in a way that does not depend on `cl' package).

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>