struct RGB for Rust

Rust is strongly typed and your RGB pixel is not compatible with my RGB pixel.

So let's all use mine :P

```rust extern crate rgb;

let px = RGB{r:255u8,g:0,b:100}; asserteq!(px.as_bytes()[0], 255);

let px = RGB8::new(255,0,255); let inverted = px.map(|ch| 255 - ch);

println!("{}", inverted); // rgb(0,255,0) ```