Improve scroll zoom handling (#50)

* decrease the step size of each zoom operation

* use same stepping on all browsers

* replace math.sign with polyfill

client-data/js/board.js

 		clearTimeout(scrollTimeout);
 		scrollTimeout = setTimeout(function updateHistory() {
 			var hash = '#' + (x | 0) + ',' + (y | 0) + ',' + Tools.getScale().toFixed(1);
-			if (Date.now() - lastStateUpdate > 5000 && hash != window.location.hash) {
+			if (Date.now() - lastStateUpdate > 5000 && hash !== window.location.hash) {
 				window.history.pushState({}, "", hash);
 				lastStateUpdate = Date.now();
 			} else {

client-data/tools/zoom/zoom.js

             var x = evt.pageX / scale;
             var y = evt.pageY / scale;
             setOrigin(x, y, evt, false);
-            animate((1 - evt.deltaY * ZOOM_FACTOR / 10) * Tools.getScale());
+            animate((1 - ((evt.deltaY > 0) - (evt.deltaY < 0)) * 0.25) * Tools.getScale());
         } else {
             window.scrollTo(window.scrollX + evt.deltaX, window.scrollY + evt.deltaY);
         }