Added disable_authenticity_token option to form helper

Hello.

I've added a new option to the form helper. With this option we can
generate forms without authenticity_token tag.
It is useful when we generate forms for some external resources like
payment gayteways where names of fields are often restricted so forms
with authenticity_token are not valid.

Usage example: form_for(@survey, :disable_authenticity_token => true)

Ticket with patch with tests:
https://rails.lighthouseapp.com/projects/8994/tickets/6228-added-disable_authenticity_token-option-to-form-helper