Made excel export format enabled by default
and added PHPExcel as a suggested library through composer

If PHPExcel is not installed, the excel export format will not be available
Improve check for PHPExcel being loaded

Before a file with references to non-loaded code would still get parsed,
that is not very nice.
Style improvements to excel format code
Merge pull request #22 from SemanticMediaWiki/enableexcel

Enable the excel format by default is it can be loaded