Import bullet by submodule rather than directly importing into a folder

[cornytrace]

Using git submodule you can add other git projects in a subdirectory of your own git project. This is preferrable over directly importing into a folder as any update to the bullet project will just require one changed line in the .gitmodules file. Furthermore, when you already have bullet on your computer you can just copy the folder over without any problems as you need to do a manual git submodule update --init to clone bullet the first time you clone the gamekit repo onto your computer. It will save a lot of disk space and bullet related git messages in our git commit messages.

[erwincoumans]

Only once the github download button includes all submodules. One of the goals of gamekit is to make it easy to get started.