Add option to send additional fields with the auto complete query.

Hi!

In many cases you wish to send additional fields with the auto complete query.  An example can be that you first select a project, and then use auto complete for a work record description text field, and you wish to prioritize previous work records from the same project before those from other projects.

This pull request adds a "fields" option to the FormHelper.  The option is expected to contain a Ruby hash with names of query params as keys and CSS selectors for the referenced fields as values.

```
<%=form_for @work do |f| %>
  <%=f.autocomplete_field :description, '/works/auto_complete_work_description',
      :fields => {:project_id => '#work_project_id'}%>
<% end %>
```

The change should be backwards compatible and not affect any legacy code.  Please merge and include in the next gem release.