Added opening hours validator.

Hi

Nice validation plugin. I tried to add support for opening_hours but I have no idea how to debug it in JOSM. I did not find a error console or something like this. Next thing is that I do not run any logic or scripts on server side to evaluate opening_hours errors. So my approach for this plugin is simply to download all opening_hours in one area and check them with [pyopening_hours](https://github.com/ypid/pyopening_hours) for problems on the client.

I wrote a proof of concept. If you like it could you please adopt the API of qat_script for this and get it to work in JOSM to get me started?