Created main menu scene and pause screen prefab

[Ste-Gar]

All visuals are placeholder. This PR adds:

• a main menu scene with an options menu (sound options; resolution and fullscreen options are available but disabled as they are unnecessary for a WebGL build).
• a PauseCanvas prefab which can be added to any scene; it includes a game manager that pauses the game.
• a Scene Loader that assists in loading scenes.
• a Fader prefab, which allows the scene loader to apply a fade out or fade in effect when loading scene.
• an AudioMixer set up with 3 channels.
• SoundManager and SoundClip scripts that can be used to manage sound playback

[Astrydax]

Awesome looks good.