Register gettext callables for the jinja2 renderer

As a workaround currently the following code seems to work:

```python
def install_gettext_callables(app):
    from tg.i18n import ugettext, ungettext
    jinja2_env = app_globals.config['pylons.app_globals'].jinja2_env
    jinja2_env.install_gettext_callables(ugettext, ungettext)
    return app

base_config.register_hook('after_config', install_gettext_callables)
```

This patch has not been tested yet due to missing testing environment though. I'd be glad if someone could help me with that...