Fixed small bug in isFirstSlide

The function ```isFirstSlide``` returns false when the **first** slide is visited immediately after a vertical slide.


**Steps to reproduce:**

1- Create a structure similar to this:

```
<div class="slides">
<section>
    <h1>First</h1>
</section>

<section>
	<section>
		<h1>1/0</h1>
	</section>
	<section>
		<h1>1/1</h1>
	</section>
</section>
</div>

```

2- Go to 1/1

3- Go to First

4- Call ```Reveal.isFirstSlide( )```


**Actual behavior:**

```Reveal.isFirstSlide( );``` returns **false** when in first slide.


**Expected behavior:**

```Reveal.isFirstSlide( );``` returns **true** when in first slide.