Added makefiles, fixed Pacejka, other fixes and improvements.

- Added experimental makefiles.
- Finally fixed Pacejka / slip ratio simulation at low speeds.
- Added switch to choose simulation mode.
- Fixed bugs with neutral gear position.
- No longer reset auto-gear switch when pressing 'R'
- Improved syncing between engine RPM and wheel angular speed. Now it's
no longer instantaneous.
- Less air drag for bikes.
- Fixed rolling resistance too strong.
- Added simple downforce computation.
- Added tire load sensivity to strafe speed.
- Improved options menu ui.
- Improved vehicle selection screen.
- Removed topdown example from all build config. Create specific build
configs for it.
- Refactorings.