Fixed Seo Twig Extension

- Replaced deprecated Twig features
- Introduced safe html escaping
- Changed echo to return

I've kept the old functions to just output the return value of the new functions. This means it's still BC when calling the `render*` functions. It is also BC when using in twig, since you had to put the function calls in `{{ ... }}` tags, these will always output the return value. At last, it allows you to tweak the returned text. For instance:

````twig
<title>{% block title %}{{ sonata_seo_title()|strip_tags }}{% endblock %}
````

There is one little BC break: If someone tried to change the behaviour of one the Twig functions by extending this class and overriding one of the `render*` methods, they should not override the `get*` methods instead.