submit form helper should use i18n for text

Sometimes it's useful to have default texts for submit buttons, specially in admin interfaces.
The attached patch adds the possibility of using i18n to setup default text for the submit form helper. The submit helper look for translations based on object status (ie :new_record?), using :create or :update keys, or the default :submit key when no object exists. The model human name is always used in translation as well.