[#2847] Added Deliverable#overhead_spent to calculate the overhead cost so far.

lib/overhead_deliverable_patch.rb

@@ -0,0 +1,26 @@
+require_dependency 'deliverable'
+
+module OverheadDeliverablePatch
+  def self.included(base)
+    base.send(:include, InstanceMethods)
+
+    base.class_eval do
+      unloadable
+    end
+  end
+
+  module InstanceMethods
+    # Cost of time logged to overhead activities
+    def overhead_spent
+      time_logs = issues.collect(&:time_entries).flatten
+
+      return time_logs.collect {|time_entry|
+        if time_entry.billable?
+          0
+        else
+          time_entry.cost
+        end
+      }.sum 
+    end
+  end
+end