added getActiveStep to api

api.getActiveStep() returns the active element, or an empty div if api.init() has not been called.

I wanted to call a setTimeout on api.next() unless the last slide in my presentation is active:

```javascript
function doNext() {
    api.next();
    if (api.getActiveStep().id !== 'my-last-step') {
        setTimeout(function(){
            doNext();
        }, 2000);
    }
}
```

This would also allow you to do things with custom data attributes:
```
<div id="impress">
    <div class="step" data-time-to-next-transition="5000">hello</div>
</div>
<script>
var timeToNextTransition = api.getActiveStep().dataset.timeToNextTransition;
setTimeout(function(){
    api.next();
}, timeToNextTransition);
</script>
```