Asset pipeline integration

Hi!

I've recently added asset pipeline integration into Serious. Now Serious supports following features:

* Asset pipeline (assets dir with stylesheets/ images/ and javascripts/)
* Stylesheets with scss syntax
* Plain js or coffeescript
* Assets helpers (asset-path and asset-url) inside your scss files
* HTML5 layout with html5-boilerplate features

All tests are passed. I didn't add any additional tests. I think we can to add some tests for assets in future.