[#1147] Added comments for what things are for.

players/youtube/popcorn.youtube.js

@@ -7,6 +7,8 @@ window.onYouTubePlayerAPIReady = function() {
   }
 };
 
+// existing youtube references can break us.
+// remove it and use the one we can trust.
 window.YT = null;
 
 onYouTubePlayerAPIReady.waiting = [];

players/youtube/popcorn.youtube.unit.html

@@ -4,6 +4,8 @@
   <title>Popcorn YouTube Player</title>
   <link rel="stylesheet" href="../../test/qunit/qunit.css" type="text/css" media="screen">
   <script src="../../test/qunit/qunit.js"></script>
+
+  <!-- Tests the safe-load of Youtube library -->
   <script src="http://s.ytimg.com/yt/jsbin/www-widgetapi-vflCAfh6H.js"></script>
   <!--
     do not move - this must be called immediately prior to popcorn.js.