[merge request] customizable lisp dir (Hooray!)

[Teika Kazura <teika lavabit com>]

Hi. Please merge my branch. It includes:

* Customizable lisp directory

User can add lisp directories to be read with the environmental
variable SAWFISH_USER_LISP_DIR. Its value is directories separated by
colon, like PATH, and prepended to load-path. If not set,
~/.sawfish/lisp is assumed.

User can override system lisp files. For example, if there is
~/.sawfish/lisp/sawfish/wm/windows.jl, then it is read as
sawfish.wm.windows, instead of the installed one. In fact, it was
possible for some lisp files, but now it's guaranteed for all files.
Of course, you can define new modules, too.

This is extremely useful for developers. In my case, 
SAWFISH_USER_LISP_DIR=~/.sawfish/overlay:~/.sawfish/lisp .
The former is for testing, and the latter for my own settings.
When you've got files to test, stash 'em into the "overlay". When
you're done, then remove all files in the overlay. (It's good to
keep the directory structure.)

* Some doc patch.

Teika++,
Teika (Teika kazura)