Support writing "Creating application" property in Excel5

[maxxer]

Some files can contain a property called "Creating application".
Gnumeric calls it meta:generator.
There's a reference to this property in Classes/PHPExcel/Reader/Excel5.php, row 1326, but it's actually not supported.
It would be great to be able to write this tag to XLS files.

[maxxer]

I've a demo file available on Dropbox.
The tag in that file says VRU-NAB-1.5.0.2.