Create global functions to require Javascript and CSS files

Not every page requires every CSS file, and the same thing goes for JS files. A global function set `require_js` and `require_css` should be created which can be called at any point of execution by the controller in the format:

``` hack
require_js('/js/application.js');
require_css('/css/application.css');
```

which create css/ js require tags in the top-level renderer.
