Add respond_to? method definition for search panes

In my application I am relying on ducktyping to display excerpts only if they are available (i.e. when thinking-sphinx was used to get a list of results):

```erb
<% if result.respond_to?(:excerpts) %>
  <%= result.excerpts.title.html_safe %>
<% else %>
  <%= result.title %>
<% end %>
```

"To have a consistent API when using method_missing, it’s important to implement a corresponding respond_to?." (source: http://technicalpickles.com/posts/using-method_missing-and-respond_to-to-create-dynamic-methods/)