Remove Dependency on JExcelAPI

[benoitx]

The ExcelTransformer is the only class in Flatpack depending on JExcelAPI.

I suggest that we move that class into a 'flatpack-excel' module.

[benoitx]

Suggest that we change the ExcelTransformer to be a Writer.

[martindiphoorn]

If it is being rewritten then maybe think about using an other library.
JExcelApi latest version is from 2009 and there site is dead, only sourceforge is a live.

I would suggest using apache poi.

[martindiphoorn]

Just added a simple poi example which can be easily extended. Be aware that i needed an outputstream so i hacked it into the factory.

See this commit:
martindiphoorn@0363320