add polyfill for Math.sign.

js/pixl/public/index.html

@@ -42,6 +42,16 @@
			ev.movementY = ev.movementY || ev.mozMovementY || ev.webkitMovementY;
			return ev;
		};
        Math.sign = Math.sign || function(n) {
            if (n < 0) {
                return -1;
            } else if (n > 0) {
                return 1;
            } else {
                return 0;
            }
        };
	</script>
	<script>
		window.pixl = {};