Add a new Submodule module and class which encapsulates a git submodule and allows it to be handled.

The class needs to be designed first based on the current implementation, but feel free to improve on the existing git submodule design if possible.

When working on it, don't forget to update the Tree class to return your submodule type when it is being traversed ( see git.objects.Tree._iter_from_data ).

Note: This ticket involves quite some amount of work, and it might be worth splitting it into sub-tasks.