Add "Write ppm file" and "Bitmap."

Implements http://rosettacode.org/wiki/Write_ppm_file and http://rosettacode.org/wiki/Bitmap/.
You can also pattern match in the function declaration, to get the x and the y:

```
fn index_mut<'a>(&'a mut self, &(x, y): &(uint, uint)) -> &'a mut Color {
```