Replace current design with the design from Piston

[bvssvni]

I think we should make a gradual change from the Piston design to make it more flexible.

First we can start to move over things like Mouse and Keyboard.

[bvssvni]

Next thing I would like to move over is the Args structures. These define the data that goes with different events.

[bvssvni]

tomaka suggested to separate the Render and Update from Input, which will give the new structure:

pub enum GameEvent {
    Render(RenderArgs),
    Update(UpdateArgs),
    Input(InputEvent)
}