Homogenization of writer Excel5 with Excel2007.
PHPExcel object is now optional and use setPHPExcel and getPHPExcel.
Merge branch 'WriterExcel5OptionalConstructor' of https://github.com/myclabs/PHPExcel into WriterExcel5OptionalConstructor
Merge branch 'WriterExcel5OptionalConstructor' of https://github.com/myclabs/PHPExcel into WriterExcel5OptionalConstructor