add support for the @group annotation at the class level

[michelv]

This change brings support for including/excluding whole test classes with the @group annotation and the <groups> tag in the PHPUnit XML configuration file.

The patch applies cleanly to both master and the tag v1.6.1.

[DonCallisto]

Hi @michelv thanks for your contribution.
As you correctly stated in #142

The drawback is that if the annotation @group is used only on some of the tests in a testsuite, the testsuite will still be executed entirely.

I think that, as this will be a new "feature", you should try to implement this functionality in its entirety so you should also take into account the @group annotation at test level.

If you try to take a look at this and modify accordingly it would be great.
If you find some issue let me know and we can dig our toes together inside the code (you know, I've not started the project so I can't say I know its code by heart).