[merge request] window-class ext, window-pid, doc.

[Teika Kazura <teika lavabit com>]

Hi, please merge my branch which includes:
* window-class accepts new option 'spec', so that return value
  ("emacs" . "Emacs") and "Emacs/emacs" are possible.
* window-pid function.
* doc.

Chris wrote:
> It's working here (and not breaking anything), so I'll commit it,
> though I would like to have some more tests before release this
> time. (I've got 96 .desktop files, but none hit those corner cases).

Thanks. Testing is indeed desirable, since it includes code
cleanups. (I couldn't seperate it to fix the whitespace bug, and
without tidying, 'R' fixing was messy.)

Regards,
Teika (Teika kazura)