Support of ButtonType

Symfony 2.3 added the ButtonType in form. It generate a
Twig_Error_Runtime exception when using form_javascript and/or
from_stylesheet cause there is no button_javascript or
button_stylesheet.
Add them

See https://github.com/genemu/GenemuFormBundle/commit/2ec24249a4f246021a62e6cf5c23a689ee2d5b9a