Full asset pipeline support for themes, remove themes_for_rails [THEME]

Remove themes_for_rails gem as it's no longer being maintained by its owner,
doesn't utilize the asset pipeline and can be replaced by adding a couple of
paths to the asset pipeline configuration (config/initializers/themes.rb).

[THEME] Views work exactly as before. There is no longer a theme_ prefix for
asset helpers (use stylesheet_link_tag instead of theme_stylesheet_link_tag).
You will also want to rewrite your asset helper methods to take advantage of
the asset pipeline.

  In themes/*/views/layouts/application.html.erb, change:
    <%= theme_stylesheet_link_tag 'typography', 'forms', 'layout', ... %>
  to look like:
    <%= stylesheet_link_tag 'theme', :media => :all %>
  and require your assets from themes/*/stylesheets/theme.css:
    /*
     *= require typography
     *= require theme-forms
     *= require layout
     * ...
    */

Note that, just as with views, theme-specific assets now override any core
application asset with the same name; it's best to avoid names already
being used in app/assets/*/ (i.e. forms.js) unless you want to explicitly
replace their behavior, not just augment it.

You should also ensure any theme-specific javascript and stylesheet assets
referenced by an asset helper method (by your views, not within the asset),
are also defined in your theme/*/settings.yml file (see default theme):

  precompile_assets:
    - 'mobile.css'
    - 'print.css'

You can verify your theme will look correct on production by running:

  $> bundle exec rake clear assets:precompile
  $> RAILS_ENV=preview bundle exec rails s -blocalhost

And walking through your application, keeping an eye for any 404 errors.
Rename .css files to .scss to support asset helpers [DEPLOY]

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.

[DEPLOY] 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 [DEPLOY]

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).

[DEPLOY] 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") ...