fix basic asset host

lib/middleman/features/asset_host.rb

@@ -1,22 +1,32 @@
 module Middleman::Features::AssetHost
   class << self
     def registered(app)
+      app.set :asset_host, nil
+      
       app.compass_config do |config|
-        if app.asset_host.is_a?(Proc)
-          config.asset_host(&app.asset_host)
+        if self.asset_host.is_a?(Proc)
+          config.asset_host(&self.asset_host)
         end
       end
-
-      app.register_asset_handler :asset_host do |path, prefix|
-        original_output = self.before_asset_handler(:asset_host, path, prefix)
-
-        valid_extensions = %w(.png .gif .jpg .jpeg .js .css)
-
-        asset_prefix = self.asset_host.call(original_output)
-
-        File.join(asset_prefix, original_output)
-      end
+      
+      app.send :include, InstanceMethods
     end
     alias :included :registered
   end
+  
+  module InstanceMethods
+    def asset_url(path, prefix="")
+      original_output = super
+
+      valid_extensions = %w(.png .gif .jpg .jpeg .js .css)
+
+      asset_prefix = if self.asset_host.is_a?(Proc)
+        self.asset_host.call(original_output)
+      elsif self.asset_host.is_a?(String)
+        self.asset_host
+      end
+
+      File.join(asset_prefix, original_output)
+    end
+  end
 end