FEATURE: Native theme support

This feature introduces the concept of themes. Themes are an evolution
of site customizations.

Themes introduce two very big conceptual changes:

- A theme may include other "child themes", children can include grand
children and so on.

- A theme may specify a color scheme

The change does away with the idea of "enabled" color schemes.

It also adds a bunch of big niceties like

- You can source a theme from a git repo

- History for themes is much improved

- You can only have a single enabled theme. Themes can be selected by
    users, if you opt for it.

On a technical level this change comes with a whole bunch of goodies

- All CSS is now compiled using a custom pipeline that uses libsass
    see /lib/stylesheet

- There is a single pipeline for css compilation (in the past we used
    one for customizations and another one for the rest of the app

- The stylesheet pipeline is now divorced of sprockets, there is no
   reliance on sprockets for CSS bundling

- CSS is generated with source maps everywhere (including themes) this
    makes debugging much easier

- Our "live reloader" is smarter and avoid a flash of unstyled content
   we run a file watcher in "puma" in dev so you no longer need to run
   rake autospec to watch for CSS changes

lib/stylesheet/compiler.rb

@@ -0,0 +1,60 @@
+require_dependency 'stylesheet/common'
+require_dependency 'stylesheet/importer'
+require_dependency 'stylesheet/functions'
+
+module Stylesheet
+
+  class Compiler
+
+    def self.error_as_css(error, label)
+      error = error.message
+      error.gsub!("\n", '\A ')
+      error.gsub!("'", '\27 ')
+
+      "footer { white-space: pre; }
+      footer:after { content: '#{error}' }"
+    end
+
+    def self.compile_asset(asset, options={})
+
+      if Importer.special_imports[asset.to_s]
+        filename = "theme.scss"
+        file = "@import \"#{asset}\";"
+      else
+        filename = "#{asset}.scss"
+        path = "#{ASSET_ROOT}/#{filename}"
+        file = File.read path
+      end
+
+      compile(file,filename,options)
+
+    end
+
+    def self.compile(stylesheet, filename, options={})
+
+
+      source_map_file = options[:source_map_file] || "#{filename.sub(".scss","")}.css.map";
+      engine = SassC::Engine.new(stylesheet,
+                                 importer: Importer,
+                                 filename: filename,
+                                 style: :compressed,
+                                 source_map_file: source_map_file,
+                                 source_map_contents: true,
+                                 theme_id: options[:theme_id],
+                                 load_paths: [ASSET_ROOT])
+
+
+      result = engine.render
+
+      if options[:rtl]
+        require 'r2'
+        [R2.r2(result), nil]
+      else
+        source_map = engine.source_map
+        source_map.force_encoding("UTF-8")
+
+        [result, source_map]
+      end
+    end
+  end
+end