Recursive submodule update

[jlehmann]

@hvoigt and I are going to work on https://github.com/jlehmann/git-submod-enhancements/tree/recursive_submodule_checkout and stuff related to that, anyone interested is invited to join that effort. We'll also be happy to help if someone wants to tackle other topics from the "still to do" list.

[hvoigt]

Put me in the participators list...