Introduce plugin framework

* Source files are under src/
* js/impress.js is now generated, but remains part of the repo (so it just works)
* npm run build
* build.js uses buildify node module
* Break out navigation and resize plugins from core src/impress.js file

src/plugins/resize/resize.js

@@ -0,0 +1,47 @@
+/**
+ * Resize plugin
+ *
+ * Rescale the presentation after a window resize.
+ *
+ * Copyright 2011-2012 Bartek Szopka (@bartaz)
+ * Released under the MIT license.
+ * ------------------------------------------------
+ *  author:  Bartek Szopka
+ *  version: 0.5.3
+ *  url:     http://bartaz.github.com/impress.js/
+ *  source:  http://github.com/bartaz/impress.js/
+ *
+ */
+
+/* global document, window */
+
+( function( document, window ) {
+    "use strict";
+
+    // Throttling function calls, by Remy Sharp
+    // http://remysharp.com/2010/07/21/throttling-function-calls/
+    var throttle = function( fn, delay ) {
+        var timer = null;
+        return function() {
+            var context = this, args = arguments;
+            window.clearTimeout( timer );
+            timer = window.setTimeout( function() {
+                fn.apply( context, args );
+            }, delay );
+        };
+    };
+
+    // Wait for impress.js to be initialized
+    document.addEventListener( "impress:init", function( event ) {
+        var api = event.detail.api;
+
+        // Rescale presentation when window is resized
+        window.addEventListener( "resize", throttle( function() {
+
+            // Force going to active step again, to trigger rescaling
+            api.goto( document.querySelector( ".step.active" ), 500 );
+        }, 250 ), false );
+    }, false );
+
+} )( document, window );
+