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".

The existing interactive rebase machinery is extended to handle "git am"
mailbox applications.  Git shares structure between "git rebase" and
"git am", (e.g., they both use the .git/rebase-apply directory) so this
is a natural extension.
Merge branch 'pr/git-am'