Added night mode

As a suggested enhancement here: https://github.com/gabrielecirulli/2048/issues/146
I have created a small javascript to alter the background colour to dark which makes the game much more comfortable to play at night.

A small button with a moon icon has also been added which facilitates the switching between the normal and night mode. I've tested the game and the added javascript has caused no recognizable obtrusion in the normal functioning.