Support relative links with search_index_path

Using search_index_path I always got the absolute path back but for hosting on github a relative path is needed.

I wrote my own helper which uses link_to to get the relative path.

``` ruby
def link_to_search_index
  absolute_link_to_index = '/' + extensions[:search].options[:index_path]
  dom = Nokogiri.HTML(link_to '', absolute_link_to_index)
  node = dom.at_xpath '//a'
  node[:href]
end
```
