Added motorway track

Included a large oval track which is representative of motorway driving
(large corners, shallow bends). To install, copy the Motorway folder in to
runtime/tracks/road
Set default sample time to 0.02

Removed dependancy on matching TORCS physics model to Simulink base rate.
Now defaults back to TORCS robot update rate (50Hz).
Fixed bug in non-restart code

Updating the position field of the vehicle structure with restarts
disabled was using the old format of structure, this commit fixes that
Added TORCS patches

- Added patch for the tgf.h file which adds a double precision position
  structure
- Added patch for the car.cpp file which uses double precision position
  variables
Modified example model

Removed the step input for time headway and replaced with a constant. This
keeps the target constant for longer during the end of the simulation,
allowing the floating point quantisation problem to be observed (albeit in
a very minor way).
Readme added

Readme added which desribes...

- Installation of TORCSLink plugin within TORCS
- Usage of example ACC model
- Modification of TORCS to enable restarts and double precision position
  values