Don't check the type of fn before running it -- let this cause an exception instead
Call Camera.addDrawing() with the object instead of its function and
position separately.

Before this, it was not obvious what the function and position that
addDrawing was being called with were.
Combine Camera.prototype.setup and its constructor
Removed unused this.position in Camera
Remove ternary if, and use === instead of ==
Remove duplications in drawMines, drawLife in HUD
Make Item.makeSprite more functional and remove duplicated code