use proper section headings to separate packages

Instead of e.g.

   ;; package

use

;;;;; package

This allows collapsing sections similar to how this is done in Org
mode.  The outshine minor-mode can be used to achieve them same in
Emacs-Lisp and other major-modes.
mention rainbow-mode above color palette
remove semicolon-only lines around commentary
remove empty line at end of package sections
teach rainbow-mode about zenburn colors

Teach Rainbow mode to also add font-lock keywords for Zenburn color
identifiers (symbols but actually all text occurances).

* zenburn-colors-alist: hold list of zenburn colors and their values
* zenburn-with-color-variables: macro to let bind these color symbols
* zenburn-add-font-lock-keywords: whether to enable add zenburn
  keywords in all elisp buffers, by default only in zenburn-theme.el
* zenburn-turn-on-rainbow-mode: turn on rainbow-mode if it is
  available.

The activation of rainbow-mode has been improved.  Users now only get
a message about a potentially unsave eval form if they have neigher
loaded nor autoloaded zenburn-theme before visiting the file.
Merge pull request #105 from tarsius/master

For easier navigation and hiding: use proper section headings to separate packages
Merge pull request #106 from tarsius/rainbows

 teach rainbow-mode about zenburn colors