git-blame: Add Emacs Lisp file headers and GNU GPL boilerplate

git-blame: Add Emacs Lisp file headers and GNU GPL boilerplate

Add Emacs Lisp file headers, according to "Coding Conventions" chapter
in Emacs Lisp Reference Manual and Elisp Area Convetions for
EmacsWiki:
  http://www.emacswiki.org/cgi-bin/wiki/ElispAreaConventions
Those include: copyright notice, GNU GPL boilerplate, description and
instalation instructions as provided in email and in commit message
introducing git-blame.el, compatibility notes from another email by
David Kågedal about what to change to use it in GNU Emacs 20, and
"git-blame ends here" to detect if file was truncated.  First line
includes setting file encoding via first line local variable values
(file variables).

Added comment to "(require 'cl)" to note why it is needed; "Coding
Conventions" advises to avoid require the `cl' package of Common Lisp
extensions at run time.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>