added method to disable the SlotMachine

this method would remove the event listener from the start button
this method would be helpful if the consumer want to use the same markup
for a different slot machine.

src/slot-machine.js

@@ -275,6 +275,14 @@ function SlotMachine(config) {
     this.startButton.addEventListener('click', start);
   };
 
+  /**
+   * @function disable
+   * disable the instance of a SlotMachine
+   */
+  this.disable = function() {
+    this.startButton.removeEventListener('click', start);
+  };
+
 }
 
 module.exports = SlotMachine;

test/slot-machine.js

@@ -248,6 +248,12 @@ describe('SlotMachine', function() {
       }, 5000);
     });
 
+    it('should disable the slotMachine', function() {
+      var spy = sinon.spy(slotMachine, 'disable');
+      slotMachine.disable();
+      assert.isTrue(spy.called);
+    });
+
     // it('should result in Jackpot, for tea', function(done) {
     //   this.timeout(10000);
     //   slotMachine.reels.forEach(function(reel) {