Register GoogleMapType as a service

Hi @ollietb, 
I recently used your bundle and I think GoogleMapType must be register as a service.

Before:

     use Oh\GoogleMapFormTypeBundle\Form\Type\GoogleMapType;
     [...]
    $builder->add('latlng', new GoogleMapType());

After:

    $builder->add('latlng', 'oh_google_maps');

Thanks !