Support lazy django templates rendering (DjangoMessage)

[lavr]

Now we have to make explicit django template rendering:

    from django.template.loader import get_template
    DjangoMessage(html=get_template(template_name).render({'a': 42}, ...)

Proposed way is to support lazy rendering:

    m = DjangoMessage(html=get_template(template_name)
    m.render({'a': 42}, ...)