Mark all failing unit tests to be skipped

This allow us to start on a clean state, to progressively fix existing
failing tests and add new tests easily. All of that while ensuring that
we don't introduce regressions.

I would recommend merging this as soon as possible to have clean state for further development.

Also, as you can this on my Travis (https://travis-ci.org/PowerKiKi/PHPExcel/builds/15840389) PHP 5.2 is the only version that fails. I would drop support for that version by modyfing `.travis` file. But that may be too drastic and not in line with PHPExcel roadmap ...