Package.el merged

[Chong Yidong]

I have now added package.el, Tom Tromey's package system, to the trunk.
It's been discussed on this list previously; to summarize, it provides a
convenient way to download and install Emacs Lisp packages and have them
loaded automatically at startup.

One change, relative to package.el 0.9, is that package-archive-base now
points to elpa.gnu.org, a server that the FSF has graciously provided
for our use.  This is currently mostly empty, except for a couple of
test packages.  Let's concentrate on polishing the packaging system
before worrying about populating it.

Two defcustoms, `package-enable' and `package-load-list', allow the
package system to be disabled, or for only specific packages to be
loaded, or disabled, or "held" at fixed versions.  The default setting
is to load all installed packages automatically at startup, after
reading the init file.

Currently package.el is not in loadup.el; I am still unsure whether to
add it, or use autoloads and juggle things so that it's not loaded
unless the user installs a package.

There is still a great deal of polish that can be added, such as menu
bar integration and improvements to the *packages* buffer (currently not
very user friendly IMO).  Also, Stefan has requested a way to make the
way packages are loaded during start-up more transparent, to encourage
users to customize it; I have some ideas about how to accomodate this
within the current system, and am working on them.

Regardless, I think it is good to check the code into the trunk earlier
than later, so here it is.  Go ahead and poke at the system, find flaws,
etc.


Also: if anyone among the active Emacs contributors is willing to help
flesh out the elpa.gnu.org website, please email me!