Revert "Removes PHPExcel in favor of composer managed one"

This reverts commit 5cadb8424877288af0ee3668cd5a37cbf07de475.
Uses composer not for managing PHPExcel any more
The composer autoloader implementation makes it really hard to run the library tests also in applications that embed the Arcavias core
Merge pull request #228 from nsendetzky/master

Uses composer not for managing PHPExcel any more