Streamlined calendar/lang

I opened a [ticket](http://yuilibrary.com/projects/yui3/ticket/2532916) about these changes a looong time ago, but never got any feedback from Allen (I contacted him several times via the forums). I ran into Calendar weirdness again lately and decided to just clean this stuff up. See the individual commits' messages for details.

There's still more left to do, however:
1. `lang/calendar-base.js` still has a `weekends` key, which is unused. I didn't remove it yet because it might be used to style those days differently. But if we're not going to do that, we should just remove the key.
2. `first_weekday` is used, but (as I argue in the original ticket), it might be more of a preference thing than a language / locale setting. According to [ISO 8601](http://en.wikipedia.org/wiki/ISO_8601), the week starts on Monday. But [JavaScript's Date object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date) counts days as 0-7, starting from Sunday. What to do?
3. `very_short_weekdays` are two-letter abbreviations for column headers. `datatype-date-format` has three-letter abbreviations for far more languages. I would suggest using these.

If these three keys are all removed, we can remove calendar's `lang` altogether.