Tag & indent

Hi,

These have been bugging me for a while. I finally got around to fixing them. Please let me know what you think about including them.

- Add `empty` tag as closing tag, which does the same for `for` as `else` and `elif` do for `if`.
- Add `plural` tag as indenting and closing tag, which is similar (syntax-wise) to `empty` for `for` and `else` and `elif` for `if`. It's explained [here](https://docs.djangoproject.com/en/1.5/topics/i18n/translation/#blocktrans-template-tag).