Manager : fix off method

When you didn't attach an event and you use a Hammerjs.Manager.off
method , you will see script error.

Fixes #768
Closes #839
Ref #770

src/manager.js

@@ -206,7 +206,7 @@ Manager.prototype = {
             if (!handler) {
                 delete handlers[event];
             } else {
-                handlers[event].splice(inArray(handlers[event], handler), 1);
+                handlers[event] && handlers[event].splice(inArray(handlers[event], handler), 1);
             }
         });
         return this;

tests/unit/test_events.js

@@ -47,3 +47,15 @@ test('test the eventemitter', function() {
     ee.emit('testEvent2', inputData); // doenst trigger a thing
     ee.emit('testEvent3', inputData); // doenst trigger a thing
 });
+
+/*
+ * Hammer.Manager.off method : exception handling
+ */
+test("When Hammer.Manager didn't attach an event, 'off' method is ignored", function() {
+    var count = 0;
+    hammer = new Hammer(el, { inputTarget: document.body });
+    hammer.off("swipeleft", function(e) {
+        count++;
+    });
+    ok(true, "nothing");
+});