[Sass] Use Tree::Node#render to shorten tree rendering.

lib/sass/engine.rb

@@ -105,7 +105,7 @@ module Sass
 
     # Processes the template and returns the result as a string.
     def render
-      to_tree.perform(Environment.new).to_s
+      to_tree.render
     end
 
     alias_method :to_css, :render

lib/sass/plugin.rb

@@ -71,8 +71,7 @@ module Sass
 
       filename = template_filename(name, template_location)
       result = begin
-                 Sass::Files.tree_for(filename, engine_options(:css_filename => css, :filename => filename)).
-                   perform(Environment.new).to_s
+                 Sass::Files.tree_for(filename, engine_options(:css_filename => css, :filename => filename)).render
                rescue Exception => e
                  exception_string(e)
                end

lib/sass/tree/node.rb

@@ -35,6 +35,10 @@ module Sass
         self.class == other.class && other.children == children
       end
 
+      def render
+        perform(Environment.new).to_s
+      end
+
       def to_s
         result = String.new
         children.each do |child|