Fix mobile browser audio unlock

[Joncom]

diff --git a/lib/impact/sound.js b/lib/impact/sound.js
index 50d74be..afcc7a4 100644
--- a/lib/impact/sound.js
+++ b/lib/impact/sound.js
@@ -34,13 +34,13 @@ ig.SoundManager = ig.Class.extend({
                if( ig.Sound.enabled && ig.Sound.useWebAudio ) {
                        this.audioContext = new AudioContext();
                        this.boundWebAudioUnlock = this.unlockWebAudio.bind(this);
-                       document.addEventListener('touchstart', this.boundWebAudioUnlock, false);
+                       ig.system.canvas.addEventListener('touchstart', this.boundWebAudioUnlock, false);

                }
        },

        unlockWebAudio: function() {
-               document.removeEventListener('touchstart', this.boundWebAudioUnlock, false);
+               ig.system.canvas.removeEventListener('touchstart', this.boundWebAudioUnlock, false);

                // create empty buffer
                var buffer = this.audioContext.createBuffer(1, 1, 22050);