Moved the CSRF authenticity token to be generated on the form_tag helper
instead of form_for (the latter uses form_tag at the end anyway...).
Added tests too.
Refactored `button_to` to reuse the `form_tag` helper. Moved it to `form_helpers` to have it on the same namespace (it kind of makes sense for it to be in there) and added some tests to it as well.
Merge pull request #1115 from padrino/moved-csrf-token-to-form_tag-helper

Moved the CSRF authenticity token to be generated on the form_tag helper