magit recipe: add magit-popup.el

This file will replace magit-key-mode.el.  To prevent users ending up
with a Melpa version that misses both files due to timing issues, I am
adding this file now, then (probably tomorrow) push the commit with the
rename to Magit's repository, and then come back here and remove
magit-key-mode.el.
Merge pull request #1202 from tarsius/magit

magit recipe: add magit-popup.el