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

build.js

@@ -0,0 +1,12 @@
+var buildify = require('buildify');
+
+buildify()
+  .load('src/impress.js')
+  .concat(['src/plugins/navigation/navigation.js',
+           'src/plugins/resize/resize.js'])
+  .save('js/impress.js');
+/*
+ * Disabled until uglify supports ES6: https://github.com/mishoo/UglifyJS2/issues/448
+  .uglify()
+  .save('js/impress.min.js');
+*/