Add no_button options for modal helper

[Eric-Guo]

Sometime we just want to show modal immediately, for example when render js in AJAX jquery ujs way.

$('#bs_alarm').html('<%= j modal('Success!', id: 'mbo_modal', no_button: true) -%>');
$('#mbo_modal').modal();

[coveralls]

Coverage remained the same at 100.0% when pulling 44b19cd on Eric-Guo:no_button_for_modal_helper into 88a6398 on Fullscreen:master.