Created structure of files

* Created car_lot.rb to manage the lot(s)
* Added car.rb to create cars
* Added used_car.rb to create used cars
* Set up basic functions for CarLot class
Added structure to car.rb including depreciation
Added functionality for car.rb
Finished used_cars file and create_used_car
Added total lot value to car_lot
Added car_count method to car_lot
Added adjustment variable to change car value