Collection FormType updates

* Added additional data attributes for collections:
    * data-form-widget="collection" - defines that the field is a collection
    * data-collection-allow-delete - defines collection items can be deleted
    * data-collection-allow-add - defines collection items can be added
* Added `prototype_name` as view variable rather than accessing prototype.vars.name
* Updated .twig and php templates

Merge branch 'master' of git://github.com/symfony/symfony into feature/form-twig-prototype

Conflicts:
	src/Symfony/Component/Form/Tests/AbstractLayoutTest.php