renamed the raw tag to verbatim to avoid confusion with the raw filter

In Twig, we have a `raw` tag and a raw filter. Unfortunately, they are totally unrelated and that is confusing.

I propose to rename the `raw` tag to `verbatim`. Of course, for BC reasons, the old `raw` tag will be kept around for the foreseeable future as an alias to `verbatim`.

For the record, `raw` is used by Jinja (http://jinja.pocoo.org/docs/templates/#escaping), whereas `verbatim` was introduced in Django in 1.5 (https://docs.djangoproject.com/en/dev/releases/1.5/#verbatim-template-tag).