Add graphics framework

Add the SDL graphics framework and code to display a black 640x480
window. The window responds to the escape key to close the program.
Add keyboard handler

Add a keyboard handler to process key up and key down events, and to
keep track of pressed, released, and held keys.
Create ball class

Create the ball class with a rectangle describing the ball’s position
and size.
Draw a ball on the screen

Add a fill rectangle method to graphics for drawing white rectangles on
the screen. Update graphics.clear() to switch to black before clearing
the screen. Temporarily draw a ball in the centre of the screen.

Closes #1.
Add documentation
Merge branch 'feature/draw-ball' into develop