Rename .css files to .scss to support asset helpers [THEME]

Stylesheets now support asset helpers such as image-path and image-url.

reset.css was not renamed as it comes from a third party and shouldn't be
be modified directly.

[THEME] If you want to take advantage of asset helpers in your stylesheets
or SCSS templating in general (see http://sass-lang.com/), rename your
theme's .css files to .scss.
Convert image paths to use asset pipeline, not hard-coded [THEME]

Before the asset pipeline, views and stylesheets hard-coded the image path.
Now we can use image_path, image_tag, image-path, image-url helpers to
reference images which automatically appends an asset digest to ensure
the correct/latest version of the image is loaded (production).

[THEME] Image paths are now prefixed with /assets/ instead of /images/.
You should update any hard-coded image paths in your views and assets to
use image asset helpers.

  In your views, change elements like:
    <img src="/images/spinner.gif" alt="Spinner" ...>
  to look like:
    <%= image_tag "spinner.gif", :alt => "Spinner" ... %>

  In your stylesheets, change properties with url(...) from:
    background: url('/images/nav_marker.gif') ...
  to look like:
    background: image-url("nav_marker.gif") ...