Simplify struct creations with the Default trait

[tomassedovic]

For example, the Object::new method could be simplified to something like this:

Object {
    x: x,
    y: y,
    char: char,
    color: color,
    name: name.into(),
    blocks: blocks,
    level: 1,
    .. default::Default(),
}

Removing all the None and false values. That would simplify the tutorial as we wouldn't have to change the structs so often. The Game one seems like another likely candidate.