Implemented mute/unmute in AudioService
Fixed weapon sound bug

There was a bug that even if you didn't have enough energy to fire a
weapon it would still play the sound.
Fixed bug when shooting weapon and run out of energy

If when you run out of energy you can no longer fire a weapon. There
was a bug that caused the weapon sound to still play even though no
projectile was created. This is fixed now.
Merge pull request #172 from zackthehuman/master

Audio/volume and shooting sound bug. Closes #172 and progress on #156.