Add swipe support for navigation between steps

Also:
 - Removes the code that allowed navigation by tapping left/right edge of screen.
   - Actually, this was already removed in this branch...
 - Removes the code that disabled impress.js on mobile devices
 - Adds new API call impress().swipe()

Refactored for the plugin api from this pull request by @and3rson:
https://github.com/impress/impress.js/pull/496

Manually "cherry picked" from
c44fd0f4c1

index.html

@@ -340,7 +340,7 @@
 </div>
 <script>
 if ("ontouchstart" in document.documentElement) { 
-    document.querySelector(".hint").innerHTML = "<p>Tap on the left or right to navigate</p>";
+    document.querySelector(".hint").innerHTML = "<p>Swipe left or right to navigate</p>";
 }
 </script>