added function fromArrayExplicit

[adrianc111]

 * ex:
 * $source = array(
 *      array(
 *          array('value1', PHPExcel_Cell_DataType::TYPE_STRING),
 *          array('value2', PHPExcel_Cell_DataType::TYPE_FORMULA),
 *      ),
 *     array(
 *          array('value3', PHPExcel_Cell_DataType::TYPE_STRING),
 *          array('value4'),
 *      )
 * );