Remove direction select control from sort_select form builder

Remove direction select control from sort_select form builder and put it into its own sort_direction
helper method. This allows the two select controls to be passed different html options so that they may be styled more easily. See [issue 306](https://github.com/activerecord-hackery/ransack/issues/306). The sort_fields block will now look like:

```erb
<%= f.sort_fields do |s| %>
  <%= s.sort_select %>
  <%= s.sort_direction %>
<% end %>
```