Accept custom actions in BbTableHelper

Added method for rendering user defined actions as requested in issue #2 
They should be defined as the following:

	echo $this->BbTable->render(
			$data,
			array(
				'columns' => array(
					'id',
					'label',
					'size',
					'actions' => array(
						'items' => array(
							// Default actions begin
							'view',
							'edit',
							'delete',
							// /Default actions end
							// Custom actions begin
							'fooBarAction' => array(
								'show' => 'ss',
								'href' => array(
									'controller' => 'foo',
									'action' => 'baar'
									// NOTE: ID of the current element get automatically inserted as FIRST element by defaults definition
								),
							),
							// Custom actions end
						),
					),
				),
			)
		);