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

src/plugins/touch/touch.js

@@ -0,0 +1,68 @@
+/**
+ * Support for swipe and tap on touch devices
+ *
+ * This plugin implements navigation for plugin devices, via swiping left/right,
+ * or tapping on the left/right edges of the screen.
+ *
+ *
+ *
+ * Copyright 2015: Andrew Dunai (@and3rson)
+ * Modified to a plugin, 2016: Henrik Ingo (@henrikingo)
+ *
+ * MIT License
+ */
+/* global document, window */
+( function( document, window ) {
+    "use strict";
+
+    // Touch handler to detect swiping left and right based on window size.
+    // If the difference in X change is bigger than 1/20 of the screen width,
+    // we simply call an appropriate API function to complete the transition.
+    var startX = 0;
+    var lastX = 0;
+    var lastDX = 0;
+    var threshold = window.innerWidth / 20;
+
+    document.addEventListener( "touchstart", function( event ) {
+        lastX = startX = event.touches[ 0 ].clientX;
+    } );
+
+    document.addEventListener( "touchmove", function( event ) {
+         var x = event.touches[ 0 ].clientX;
+         var diff = x - startX;
+
+         // To be used in touchend
+         lastDX = lastX - x;
+         lastX = x;
+
+         window.impress().swipe( diff / window.innerWidth );
+     } );
+
+     document.addEventListener( "touchend", function() {
+         var totalDiff = lastX - startX;
+         if ( Math.abs( totalDiff ) > window.innerWidth / 5 && ( totalDiff * lastDX ) <= 0 ) {
+             if ( totalDiff > window.innerWidth / 5 && lastDX <= 0 ) {
+                 window.impress().prev();
+             } else if ( totalDiff < -window.innerWidth / 5 && lastDX >= 0 ) {
+                 window.impress().next();
+             }
+         } else if ( Math.abs( lastDX ) > threshold ) {
+             if ( lastDX < -threshold ) {
+                 window.impress().prev();
+             } else if ( lastDX > threshold ) {
+                 window.impress().next();
+             }
+         } else {
+
+             // No movement - move (back) to the current slide
+             window.impress().goto( document.querySelector( "#impress .step.active" ) );
+         }
+     } );
+
+     document.addEventListener( "touchcancel", function() {
+
+             // Move (back) to the current slide
+             window.impress().goto( document.querySelector( "#impress .step.active" ) );
+     } );
+
+} )( document, window );