merged branch Seldaek/markup (PR #638)

Commits
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e4590d0 Avoid creating unnecessary Twig_Markup instances, allows testing for falsiness of empty output

Discussion
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Avoid creating unnecessary Twig_Markup instances

This allows testing for falsiness of an empty output:

```jinja
{% set foo %}{% block lala %}{% endblock %}{% endset %}

{% if foo %}
   some output with {{ foo }}
{% endif %}
```

Currently this requires `{% if foo|length %}` since the set tag will always return a Twig_Markup.