Generates an Ulam Spiral/prime spiral
Displays the spiral in a window and writes the image to a PNG file.
Usage:
./primespiral size
where size is an integer > 0 that corresponds to the pixel height and width of the spiral
if size is not specified, then 540 will be used as default
Dependencies : SFML > 2.0
If SFML 2.0 is installed, then simply type make to build.