Fix rotation axis

[crizCraig]

We currently use the bike model to calculate the center (x,y), and angle of the vehicle using slip angle. However, when we rotate the four points defining the top-down ego rectangle, we use the center as the axis of rotation instead of the center of the rear axle. You can feel how unrealistic the current rotational axis is when playing the sim yourself. The bike model already accounts for the distance between the axles and center of gravity in it's x, y, and angle outputs - I believe the center of the car is modeled correctly according to the bike model - however the extents of the car and collision detection are currently off due to rotating about the center instead of the rear axle.

[crizCraig]

Being worked in in #8