Added 'FIRST' location for different headers and footers image

I spent many days trying to set two different images for headers and footers when using setDifferentFirst(true) with no success.

I then realized that there was no location set for the first image sections in Worksheet/HeaderFooter.php class. After unzipping an excel file, i just added the id's set by msoffice to the project. 

Now it has the ability to set a different image to the first headers and footers.

Usage: 

```
$objPHPExcel->getActiveSheet()->getHeaderFooter()->setDifferentFirst(true);

// for regular H&F
$objPHPExcel->getActiveSheet()->getHeaderFooter()->addImage($objDrawing, PHPExcel_Worksheet_HeaderFooter::IMAGE_HEADER_LEFT);

// for first H&F
$objPHPExcel->getActiveSheet()->getHeaderFooter()->addImage($objDrawing, PHPExcel_Worksheet_HeaderFooter::IMAGE_HEADER_LEFT_FIRST);
```