added visible property for buttons in button groups (fixes #100)

widgets/TbButtonGroup.php

@@ -77,6 +77,9 @@ class TbButtonGroup extends CWidget
 
 		foreach ($this->buttons as $button)
 		{
+            if (isset($button['visible']) && !$button['visible'] === false)
+                continue;
+
 			$this->controller->widget('bootstrap.widgets.TbButton', array(
 				'buttonType'=>isset($button['buttonType']) ? $button['buttonType'] : $this->buttonType,
 				'type'=>isset($button['type']) ? $button['type'] : $this->type,