Split .gitmodules into .anon and .ssh variants

[rektide]

Satisfying #29, this commit introduced a .gitmodules.anon and symlinks .gitmodules to it, and a .gitmodules.ssh holding the privileged addresses for the various submodules.

The result is that, by default, newcomers will be able to check out the submodules & thus benefit from storm-contrib. Users with privileges will still have the reference file available, and ready to swap in, for their special case.