submodule update feature

[kyhiden]

I used this buildpack in some applications on git.
I think it to be applicable to the applications of much git by becoming able to use a submodule.

(after git clone)

git submodule sync --recursive
git submodule update --init --recursive

There is also a need to change the following in order to delete multiple ".git".

find $BUILD_DIR -name ".git" -exec rm -rf {} \;