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

.gitignore

@@ -1,2 +1,3 @@
+/js/impress.min.js
 /node_modules
 /npm-debug.log