Add help popup plugin

Shows a help popup when user presses H. Add

    <div id="impress-help">

...to the presentation to enable it.

Other plugins send their help text to this plugin as events.

The idea and style for this help popup comes from hovercraft, which would
generate such html code into each presentation it creates.

src/plugins/help/README.md

@@ -0,0 +1,37 @@
+Help screen plugin
+===================
+
+Shows a help popup when a presentation is loaded, as well as when 'H' is pressed.
+
+To enable the help popup, add following div to your presentation:
+
+    <div id="impress-help"></div>
+
+Example CSS:
+
+        .impress-enabled #impress-help {
+            background: none repeat scroll 0 0 rgba(0, 0, 0, 0.5);
+            color: #EEEEEE;
+            font-size: 80%;
+            position: fixed;
+            left: 2em;
+            bottom: 2em;
+            width: 24em;
+            border-radius: 1em;
+            padding: 1em;
+            text-align: center;
+            z-index: 100;
+            font-family: Verdana, Arial, Sans;
+        }
+        .impress-enabled #impress-help td {
+            padding-left: 1em;
+            padding-right: 1em;
+        }
+
+
+
+Author
+------
+
+Copyright Henrik Ingo (@henrikingo), 2016
+MIT License