added view helper to include asset collections manually in the view scri...

Added view helper to have an ability to include asset collections in the view script manually.

It's usefull if I want to use conditional comments for IE stuff:
<!--[if IE 7]>
<?php echo $this->asset('login_ie7_head_js') ?>
<?php echo $this->asset('login_ie7_css', array('media' => 'screen')) ?>
<![endif]-->