Support multiple aside notes elements per slide

So far, multiple notes per slide are only supported if they are
attached to fragments (without fragments, only the first aside notes
element on a slide is displayed).  With this commit, the contents of
all notes on a slide are displayed (except for fragments with notes,
for which, as before, only each fragment's first/single note is
displayed).

js/controllers/notes.js

@@ -1,5 +1,5 @@
 /**
- * Handles the showing and 
+ * Handles the showing of speaker notes
  */
 export default class Notes {
 
@@ -89,7 +89,7 @@ export default class Notes {
 	 * Retrieves the speaker notes from a slide. Notes can be
 	 * defined in two ways:
 	 * 1. As a data-notes attribute on the slide <section>
-	 * 2. As an <aside class="notes"> inside of the slide
+	 * 2. With <aside class="notes"> elements inside the slide
 	 *
 	 * @param {HTMLElement} [slide=currentSlide]
 	 * @return {(string|null)}
@@ -101,10 +101,14 @@ export default class Notes {
 			return slide.getAttribute( 'data-notes' );
 		}
 
-		// ... or using an <aside class="notes"> element
-		let notesElement = slide.querySelector( 'aside.notes' );
-		if( notesElement ) {
-			return notesElement.innerHTML;
+		// ... or using <aside class="notes"> elements
+		let notesElements = slide.querySelectorAll( 'aside.notes' );
+		if( notesElements ) {
+			let notes = "";
+			for (let i = 0; i < notesElements.length; i++) {
+				notes += notesElements[i].innerHTML + "\n";
+			}
+			return notes;
 		}
 
 		return null;