Allowing the assets.js helper to add a single asset by it's filename

This patch adds an option to include a single css or js file via it's file name without it's extension, for example: `assets.js("/app")`. For backwards compatibility, given no file name `assets.js()` will load all files like it did before.

Coming from Rails to node I wanted a sprokets-like asset mechanism where I could add a single asset to the html and order my dependencies via `@import` or `//= require`. This patch enables the `@import` style of dependency definition in stylus. I will send a separate pull request for snocketAssets to enable `//=require` in js/cs files.

Hopefully I'm not overloading you guys with pull requests. I'm setting up my first Sails environment today so theres been a number of changes I needed to get things working with my preferred frontend stack.

Cheers,
Rob