Addet the class MusicNode extending AudioNode, setting a few defaults.

MusicNode only has one constructor which sets some default values.
Wrote comments for MusicNode.

Wrote javadoc comments for the MusicNode class.
Added String constants for some spatial names.

Added String constants:
GAME_NODE for the inGameStates main node and
LEVEL_NODE for the level node holding the different level chunks
Changed the name of inGameRootNode to gameNode.

Changed the name of inGameRootNode to gameNode.
Use the name constant LEVEL_NODE for the level node.
Changed suns color to ColorRGB.White.