Added MainMenuScene.
- Updated UIAtlas.
- Added textures for main menu.
- Added MainMenuUI component.
- Added main menu BGM.
Changed sound of button to upgrade sound.
Fixed buttons OnClick() method.
Updated MainMenuUI prefab and MainMenuScene.
Moved and updated game UI textures and prefab.
Changed HUD to use the new level label textures.
Updated UIAtlas.
Merge branch 'master' of https://github.com/scarletoath/Project-Roadkill

Conflicts:
	Assets/Scene/GameScene.unity