Menu and recharge audio

I added some sounds to the game
* a menu sound that can play over transitions to other scenes, like when going through the menus.
This way it doesn't need to be added to any other scene but the main menu
* a recharge sound for the rover which plays when appropriate and stops when recharging is done.
This needed to be included in the Main Camera Prefab, since it works with the EnergyBar script. I also added some functionality to that script so that they would work well together.
* a shutdown sound that plays when the rover runs out of energy

Fixes #10
Fixes #26