Drag todos between contexts

Closes #1020

app/views/contexts/_context.rhtml

@@ -16,4 +16,5 @@
     </div>
     <%= render :partial => "todos/todo", :collection => @not_done, :locals => { :parent_container_type => "context" } %>
   </div><!-- [end:items] -->
+  <div id="c_<%=context.id%>_target" class="context_target drop_target"></div>
 </div><!-- [end:c<%= context.id %>] -->

app/views/todos/update.js.rjs

@@ -153,3 +153,5 @@ else
   page.show 'error_status'
   page.replace_html 'error_status', "#{error_messages_for('todo')}"
 end
+
+page << "enable_rich_interaction();"