deal with strange location of emacsclient on OS X

On OS X applications are often installed in strange "executable
directories".  The Finder presents these directories as actual
applications to the user who might be blissfully unaware that
clicking on it actually executes a binary inside the directory.

So they have been happily clicking on a flashy image to start
Emacs and we come along requiring emacsclient being on $PATH.

But it gets worse.  The emacsclient executable might not even be
located in the same subdirectory of the application directory:

  /Applications/Emacs.app/Contents/MacOS/emacs
  /Applications/Emacs.app/Contents/MacOS/bin/emacsclient

So we have to look for the emacsclient here too:

  (expand-file-name "bin" invocation-directory)

Also see:

  https://github.com/magit/magit/issues/862#issuecomment-24784266