import Bullet source code from github.com/bulletphysics/bullet3 under bullet/src folder
(using 'src' folder from master branch, hash 5ed49cc3a24626bb17da5c69cc8cb758f81e91e4)
Create README.md
Update README.md
Add a proper gitignore

Directories and files for mac and linux still need to be added to this,
but it's a start.