Add vendor submodule

Add vendor submodule

People will no longer need to clone it separately, just run
`git submodule update --init`.

If people want to run composer locally and have already checked out the
submodule, a `git submodule deinit vendor` will disable the submodule.

Change-Id: I1fdaf8dc3c2a8e246bb7d2987a42a41272dffc29