Automatic observer for autocompleter prototype (in collections)

Hi.
I rewrote [my patch](https://github.com/genemu/GenemuFormBundle/pull/273) for automatic observing of *autocompleters* if they are generated from prototypes of collections. Because it was not very clean.

To render the javascript for the autocompleter you just have to insert something like 
```jinja
    {{ form_javascript(form.emails.vars.prototype) }}
```
into your twig template.