Add Paint.NET files to .gitignore

Ignore Paint.NET files.  Only need to save .png version for use in the
game.
Add a Controller class

Add a controller class.  This isolates the interpretation of user input
from the rest of a the game. It will make it easy to change control
schemes, for example.
Add nearGridLine function and fix bug

Fixed bug in Grid::snapToGrid().

Added a function Grid::nearGridLine() that returns true if a given
position is within snapping distance of a grid line.
Add car images

Added car images.  The redCarUp and redCarDown are currently
placeholders.  They need to be replaced with actual upward- and
downward-oriented images.
Add images to courier, redo control code

Added the car images to the courier.

Used all four directions (up, down, left, right) in the Courier code.
Changed how the car is controlled.  Now it is controlled with gas and
direction.  I'm not sure if I like it yet.  Also used the new Controller
class to get interpret user input.
Remove old Grid code

Removed old Grid code and added some comments describing and deprecating
old functions, which I might change my mind about and want to use later.
Add redCar{Up,Down} images

Created up and down images for the red car.  Also updated headlight
colors to a more yellow color.
Fix alignment issue with Courier

Fix an alignment issue with the Courier.  Now it is nicely aligned on
the grid lines.