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

build.js

@@ -10,7 +10,8 @@ buildify()
            'src/plugins/navigation/navigation.js',
            'src/plugins/rel/rel.js',
            'src/plugins/resize/resize.js',
-           'src/plugins/stop/stop.js'])
+           'src/plugins/stop/stop.js',
+           'src/plugins/touch/touch.js'])
   .save('js/impress.js');
 /*
  * Disabled until uglify supports ES6: https://github.com/mishoo/UglifyJS2/issues/448