Fixed 'Array to string conversion' error & various html fixes

Fixed 'Array to string conversion' error when Cron job arguments are in a multidimensional array with clear indication of the fact that the arguments are in a multidimensional array.

The issue addressed is the same as pull request #14. I had already fixed this myself before I found this repo.

The difference between pull request #14 and this fix is, that the changes in #14 will flatted the nested array and show all (grand-)children as equal, while with the fix included in this pull request, you will get an indication that the argument is an array including indenting based on array-depth.

Hope this helps & keep up the good work!

**Added second commit**:
Fixes for a number of HTML validation errors.

This pull request fixes the following html validation errors:
* duplicate id `debug-bar-cron` when on plugins page
* missing `<tr>` in `<thead>`
* old style valign/width attributes not allowed

Also makes it so css will auto-zebra-stripe the table without the need for a class name in each row