Multi value support for form fields etc.

**Bug report:**
https://groups.google.com/forum/?fromgroups=#!topic/agile-toolkit-devel/DE8U4oxz278

**Test case:**
```
class page_test extends Page {
	function init(){
		parent::init();
		
		// Main content
		$f = $this->add('Form');
		$f->addSubmit('Test');
		
		$f->add('H3')->set('ValueList');
            $f->addField('ValueList','list_single','Single');
            $f->addField('ValueList','list_multi','Multi')->setAttr('multiple');
		$f->add('H3')->set('DropDown');
            $f->addField('DropDown','dd_single','Single');
            $f->addField('DropDown','dd_multi','Multi')->setAttr('multiple');
		$f->add('H3')->set('CheckboxList');
            $f->addField('CheckboxList','cbox_single','Single');
            $f->addField('CheckboxList','cbox_multi','Multi')->setAttr('multiple');
		$f->add('H3')->set('Radio');
            $f->addField('Radio','radio_single','Single');
            $f->addField('Radio','radio_multi','Multi')->setAttr('multiple');
		
		$f->onSubmit(function($form){
            $form->js()
                ->append('<h3>Data POSTed</h3><pre>'.var_export($form->get(),true).'</pre>')
                ->execute();
		});
	}
}
```