Access session attributes from template (#492).

Within your template you can access the session object with ${session} and its attributes, e.g. ${session.username}.

Internally a Thymeleaf WebContext object is created (instead a Context object).
Thanks to Malte for raising this issue!

BREAKING CHANGES

The method for adding data to the view model is renamed:

    setViewModel(..) -> viewData(..)

See #492.