Break reliance on ActionController.

Originally brought up in #249, I didn't know how to fix this.

But now I do.

lib/draper/view_context.rb

@@ -1,8 +1,12 @@
 module Draper
   module ViewContext
+    def self.included(controller)
+      @controller = controller
+    end
+
     def self.current
       Thread.current[:current_view_context].tap do |context|
-        context ||= ApplicationController.new.view_context
+        context ||= @controller.new.view_context
         context.controller.request ||= ActionController::TestRequest.new
         context.request            ||= context.controller.request
         context.params             ||= {}