Make color::Primitive trait public

The [source code contains this line](https://github.com/PistonDevelopers/image/blob/ec4fbd02ecd4a300b3081467588fc24348dd7dce/src/image.rs#L257):

```rust
impl<T: Primitive, P: Pixel<T>> ImageBuf<P> {
```

However the hacky `Primitive` trait is private, which makes it impossible to use `ImageBuf` from outside the library.