Prevents double submission by making disable_with the default.

Default disable_with option will only be applied if user has not
specified her/his own disable_with option, whether that is in the
`data-disable-with` string form or the
`:data => { :disable_with => "Saving..." }` hash form. disable_with
will default to the value attribute.

A configuration option was added to opt out of this functionality if
the user so desires.
`config.action_view.automatically_disable_submit_tag = false`