Geefr's PPM lib

This crate provides a PPM writer, originally written for the 'ray tracing in a weekend' series.

Creating a PPM

``` use ppm::Ppm;

// Create a ppm let mut ppm = Ppm::new(width, height);

// Set a pixel // origin is top-left // r, g, b are 0-255 (u8) ppm.set_pixel(x, y, r, g, b);

// Save to a file ppm.write(String::from("foo.ppm")).expect("Failed to save PPM);

```