Update code to make slide id's optional.

Closes issue #60.

en.htm

@@ -27,7 +27,7 @@
 		<h1>Shower Presentation Template</h1>
 		<p>Vadim Makeev, Opera Software</p>
 	</header>
-	<div class="slide cover" id="Cover"><div>
+	<div class="slide cover"><div>
 		<section>
 			<header>
 				<h2>Shower Presentation Template</h2>
@@ -35,7 +35,7 @@
 			<img src="pictures/cover.jpg" alt="">
 		</section>
 	</div></div>
-	<div class="slide" id="Header"><div>
+	<div class="slide"><div>
 		<section>
 			<header>
 				<h2>Header</h2>

ru.htm

@@ -27,7 +27,7 @@
 		<h1>Шаблон презентаций Shower</h1>
 		<p>Вадим Макеев, Opera Software</p>
 	</header>
-	<div class="slide cover" id="Cover"><div>
+	<div class="slide cover"><div>
 		<section>
 			<header>
 				<h2>Шаблон презентации Shower</h2>
@@ -35,7 +35,7 @@
 			<img src="pictures/cover.jpg" alt="">
 		</section>
 	</div></div>
-	<div class="slide" id="Header"><div>
+	<div class="slide"><div>
 		<section>
 			<header>
 				<h2>Заголовок</h2>

scripts/script.js

@@ -7,6 +7,12 @@
 		l = slides.length, i;
 
 	for (i = 0; i < l; i++) {
+		// Slide ID's are optional. In case of missing ID we set it to the
+		// slide number
+		if (!slides[i].id) {
+			slides[i].id = i + 1;
+		}
+
 		slideList.push({
 			id: slides[i].id,
 			hasInnerNavigation: null !== slides[i].querySelector('.next')