Allow empty ajax search requests, add "Add new" button.

Hi,

The Following Pull request adds 2 features to the Ajax feature:
1.) Adds the capability to do "empty" search request (by inserting a space).
2.) Adds a "Add new" button to the ajax form field.

I've tested this, but i'm very new to Symfony2/Twig and so on.

Thanks for this very useful bundle :-)