Default base_path by base_url and web root dir

Default base path by base url
Is good way for many standart situation - for one server uses some upload folder in web root.
If use some file storage base path may be change.
It's remove some ugly
````
$builder->add('image', 'image', array(
   'base_path' => $this->getContainer()->getParameter('kernel.root_dir').'/../web' . '/images/',
   'base_uri' => '/images/',
));

````
or some other some code