remove bin/authors.pl and bin/mk_rel.bash

`bin/authors.pl' isn't useful anymore since we stopped having a
Copyright line for everyone who ever changed anything.

I am preparing to create a release but cannot see myself using
`bin/mk_rel.bash`, so I am removing that too.
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.
remove contrib/magit-classic-theme.el

I very much doubt that anybody is using this because it is very
incomplete.  If you do use it please contact me.

The removal is done with the okay of the original author of this
file.