User/Person models merge

app/views/conference_registration/_volunteer.html.haml

@@ -1,8 +1,8 @@
 .row
   .col-md-12
     = f.input :volunteer, :label => "Click here if you want to become a volunteer at #{@conference.short_title}", :input_html => {:maxlength => 15, :size => 40}
-    = f.fields_for :person do |p|
-      = render :partial => 'devise/registrations/volunteerperson', :locals => {:p => p}
+    = f.fields_for :user do |u|
+      = render :partial => 'devise/registrations/volunteeruser', :locals => {:u => u}
       
     %br
     - if @conference.vpositions.count > 0