add current_user_provider so people can override current_user bevior cleanly, see

http://meta.discourse.org/t/amending-current-user-logic-in-discourse/10278

lib/discourse.rb

@@ -1,5 +1,6 @@
 require 'cache'
 require_dependency 'plugin/instance'
+require_dependency 'auth/default_current_user_provider'
 
 module Discourse
 
@@ -148,6 +149,14 @@ module Discourse
     end
   end
 
+  def self.current_user_provider
+    @current_user_provider || Auth::DefaultCurrentUserProvider
+  end
+
+  def self.current_user_provider=(val)
+    @current_user_provider = val
+  end
+
 private
 
   def self.maintenance_mode_key