Simple light source and day-night cycle

There's a placeholder day-night cycle in that's just constanly
animating - it should respect turns and so forth.

Added:
* Light for defining a simple light source (to be extended w/
spotlights etc)
* nuVec3::getFloats()
* Player::getPosition()
* Graphics::enableLighting()
* Graphics::setLight()

Changed:
* Player::render() takes Light as a parameter