Implements program section for unscheduled events

close #610

app/models/conference.rb

@@ -44,6 +44,10 @@ class Conference < ActiveRecord::Base
       where(state: :confirmed)
     end
 
+    def scheduled
+      where.not(start_time: nil)
+    end
+
     def highlights
       where(state: :confirmed, is_highlight: true)
     end

app/models/event.rb

@@ -205,11 +205,11 @@ class Event < ActiveRecord::Base
   end
 
   def speaker_names
-    result = []
+    result = Set.new
     speakers.each do |speaker|
-      result.push(speaker.name)
+      result.add(speaker.name)
     end
-    result.to_sentence
+    result.to_a.to_sentence
   end
 
   private