Executes onShow promises before showing next step.

I needed the ability to perform an animation before the next tour item was shown, so I added the capability to execute a promise object on onShow. The motivating example is with a collapsible navbar in bootstrap:
```javascript
            tour.addStep({
                element: ".btn-navbar",
                title: "Navigation Menu",
                content: "This button shows the navigation menu",
                placement: "bottom",
                onShow: function(){ return $(".nav-collapse").collapse('show').promise();}
            });
        }
```