Adding possibility to use --exclude-group.

Adding exclude-group as possible command line option.

This works, since it is appended to filters that in turn are sent to PHPUnit.

(Actually, I first started to implement it in annotation filtering inside of paratest, but seeing multiple groups filtering working even with ParsedClass->getMethods falling back to return ALL methods, I decided to use the same way THAT thing is working…   …Isn't the filtering in ParsedClass->getMethods superfluous then?)