Add difficulty levels

app/views/proposal/_proposal_form.html.haml

@@ -15,6 +15,7 @@
       %br
     - else
       = f.input :event_type_id,:as => :select, :collection => @event_types.map {|x| ["#{x.title} - #{show_time(x.length)}", x.id]}, :include_blank => false, :label => "Session Type"
+    = f.input :difficulty_level, :as => :select, :collection => @conference.difficulty_levels, :include_blank => "(Please select)" if @conference.use_difficulty_levels
     = f.input :require_registration
     = f.input :abstract, :input_html => {:rows => 5, :class => "span11"},
       :required => true, :label => "Session Abstract"