Hacked some column sorting onto RatesController

* Added SortHelper to RatesController
* Init and updating the sorting (pre 0.8 sort_update)
* RatesController#index will only render the layout on non-xhr requests
* Changed Rate.history_for_user to take an order clause
* Added rate_sort_header_tag and rate_sort_link to the SortHelper in order
  to get the correct parameters for sorting Rates
* Added rate_sort_header_tags to the Rate lists

  #1915

app/models/rate.rb

@@ -11,13 +11,13 @@ class Rate < ActiveRecord::Base
   before_save :unlocked?
   before_destroy :unlocked?
   
-  named_scope :history_for_user, lambda { |user|
+  named_scope :history_for_user, lambda { |user, order|
     {
       :conditions => { :user_id => user.id },
-      :order => 'date_in_effect DESC'
+      :order => order
     }
   }
-
+  
   def locked?
     return self.time_entries.length > 0
   end