TbModal: Added support for iframe if attribute 'remote' is an absolute u...

Usage:

 ``` php 
$this->widget('bootstrap.widgets.TbModal', array(
    'buttonOptions' => array('label'=>'Yii framework'),
    'remote'=>'http://www.yiiframework.com/',
    'iframeWidth'=>600,  //default is 800
    'iframeHeight'=>300,  //default is 600
));

 ```