Changed style-related methods

Changed `Assets.php`, providing more flexibility around getting a manifest's styles. Previously one could only get a string of `<link>` tags. The new responsive *cardio* site structure requires that we get an array of the style sheets so they can be rendered programmatically.

You can see this WIP here:
https://github.com/mdx-dev/vitals/blob/cardio/style_and_wrappers/app/twig_views/elements/head_logic/head_logic.twig#L16

In summary, what's changed:

* Updated `style` method
* Introduced `style_list` method
* Introduced `gen_css_files` method used by both
* Updated documentation throughout

Note: The changes have been discussed and reviewed by @blainesch.