Make `with-editor` a standalone package

Hello,

People (other than me :stuck_out_tongue_winking_eye:) are interested in `with-editor`!

See http://emacs.stackexchange.com/questions/27/how-can-i-use-my-local-emacs-client-as-the-editor-for-remote-machines-i-access

It'd be a good idea to make it a standalone package, that way you'll get much more feedback & testing.