Move `evil-mode' definition to evil.el.

Do this so that when Emacs generates an autoload for `evil-mode', it
references the correct file to later load all of evil.

We do that so that we don't have to manually override Emacs'
generation of the ;;;###autoload cookie.

We do THAT so that we can let Emacs generate the appropriate autoloads
to enable the globalized minor mode via `custom-set-variables'.
Remove manual override to `evil-mode' docstring.

The `define-globalized-minor-mode' macro generates a docstring that
covers the same information, includes references to more functions,
and fits the standard used by many global minor modes.
Remove bindings of `evil-ret` and `evil-ret-and-indent` in insert state (fix #436).

Better use `electric-indent-mode`.
Add declaration of `evil-mode-buffers` to `evil-core.el`.
Add appending abilities to evil-write (fixes #426)