Big refactoring of registration

- Added datatable gem
- Addaed datatable to events, users (#422), registrations

app/views/conference_registrations/_volunteer.html.haml

@@ -0,0 +1,15 @@
+.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 :user do |u|
+      = render :partial => 'devise/registrations/volunteeruser', :locals => {:u => u}
+    %br
+    - if @conference.vpositions.count > 0
+      %h4
+        %u Positions that require volunteers:
+      - @conference.vpositions.each do |pos|
+        %p{:style => "font-weight:bold"} 
+          = pos.title
+          = "(#{pos.description})" if pos.description
+        
+        = f.input :vchoices, :collection => pos.vchoices.map {|x| [x.vday.day, x.id]}, :label => "Choose days to volunteer:"