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

qunit_test_runner.html

@@ -20,7 +20,7 @@
   <!-- Core tests -->
   <script src="test/core_tests.js"></script>
   <!-- Plugins -->
-  <script src="test/navigation_tests.js"></script>
+  <script src="src/plugins/navigation/navigation_tests.js"></script>
 
 </body>
 </html>