Adding class that does works conversions.

This class is meant for easing the power settings of the RaceCar's
engine power. This will let me set the power in horsepower while
grabbing that same power (work) in watts when it comes to doing the
math.
Added the engine power to the car.

I've currently hard coded the car's engine power. It might be nice to
add some flexibility to this by using setting it up so that I can pass
in an object with settings like engine power, handling, breaking power,
etc.