git format-patch action, email patches w/Emacs

"O" key-binding calls "git format-patch" from within magit.  Formatted
patches may optionally be opened in mail-mode buffers to be mailed from
within Emacs.
git-am action --- apply mbox from within magit

This patch defines the git-am action and the `magit-apply-mailbox'
function and binds them to "j" which is the *only* remaining unbound
lower-case key.