Use submodules to easy bootstrap new lesson

@jduckles was talking with me about using Git submodules to easy bootstrap new lessons. For a example, see https://github.com/coderefinery/git-intro.

[GitHub Pages supports submodule](https://help.github.com/articles/using-submodules-with-pages/)!

> If your GitHub Pages site repository contains submodules, they will automatically be pulled in when the Page is built.

# Benefits

You can use http://import.github.com/new to start a new repository with the correct files and GitHub online editor to write the lesson.

# Limitations

To edit the submodule files you will need to

1. Fork the submodule.
2. Edit `.gitmodules` to point to your fork.

# TODO

Investigate more limitations.