Ability to validate multiple select box as one field

Hello!
I needed this feature so I implemented it by myself. If you think is good enough (not much changed really) please accept this pull request. If you think the idea is good but you don't like the way I implemented, please give me some advice. If you think is a crappy idea, oh well, I'll keep it for myself.

I ran tests yesterday and returned some errors. This morning I tried the test again and doesn't give any error anymore. I noticed that sometimes it gives some errors in validation like lessThan. To be honest I never wrote tests for JS so I'm not very good at it.

If you require tests for my change I'll need more time as I can't do it at work :P 

Cheers
Vittorio