elpa branch reorganization

[Chong Yidong]

I made some changes to the setup of the elpa branch to make it more
convenient for active development.

The branch now contains a top-level Makefile.  The "make archive" and
"make archive-full" rules deploy a partial/full copy of the GNU ELPA to
the archive/ directory.  This part is similar to what we did before
using a shell script.

What's new is the "make site" rule.  This creates a site/ directory with
a layout similar to the *installed* package layout, by using symlinks.
It byte-compiles and autoloads the package sources, similar to what
happens in package installation, but without moving or copying them.

So if you're working on a package in the elpa branch, you can add the
BRANCH/site/ directory to `package-directory-list'.  Then whatever
changes you make will be visible automatically to Emacs, after a "make
site" (similar to "make recompile" when working on the main Emacs
repository.)

In terms of the branch layout, each packages now has one subdirectory of
packages/, regardless of whether it is a simple or multi-file package.
This change was necessary to allow using the packages in-place.

Feel free to suggest improvements to this system.  More work is needed,
e.g. to byte compile individual packages rather than all packages.
Sorry for any inconvenience caused by these changes, which should be
hopefully be minimal.