move script contrib/magit to bin/magit

This should make it more obvious to people who run Magit directly from
the git repository that they can add this directory to $PATH.

Until recently `bin/` contained maintenance scripts.  I hope we won't
have to add similar scripts in the future.  It should be possible to do
these things in the `Makefile' instead; provided we really need them.
move contrib/magit-{classic-theme,simple-keys}.el to separate repositories

I have created separate repositories inside the Magit organisation for
these files:

  https://github.com/magit/magit-classic-theme
  https://github.com/magit/magit-simple-keys

I don't think these files belong inside Magit proper.  Unlike
`magit-inotify.el' which is inside `contrib/' in the `master' branch
these files do not add experimental functionality which should not be
installed by default yet.  (`magit-inotify.el` depends on unrelease
Emacs-24.4).  Instead they change Magit's behaviour/looks in a way not
intended by the maintainers.

That is fine, and I think hosting the new repositories in the Magit
Github organisation is a good compromize.  Doing so has advantages for
both the people who want to use these libraries (provided my advice
about creating Elpa packages is followed) as well as me who does not
want them to complicate the build process.

Previously there was `make install_contrib' which installed both files
as well as the shell script `magit'.  I don't think many people use all
three files, so having one target to install them all makes little
sense.  I don't want to add individual targets for these files.

While it was previously possible to install these files using `make'
users who use Elpa to install Magit (and I hope most do so by now) did
not get the stuff in `contrib/' anyway.  Therefor the authors of these
libraries should add packages to Melpa and/or Marmalade if they want to
make them easily installable.

It is okay that there no longer is a target to install the shell script;
we would have to guess where on $PATH the user wants it, and would be
wrong in many cases.  It is better if users copy/move/link the script
themself.