serve assets with {{asset}} handlebars helper

You should use the `{{asset}}` handlebars helper instead of directly referencing the asset paths yourself. Read more about that [here](http://docs.ghost.org/themes/#asset-). Also, according to support I received from the Ghost folks, it's better to server the favicon from the root level of the `assets` folder. Didn't ask why, but perhaps it has something to do with nesting semantics and the way they cache things on their CDN?