Add MultipleTypeViewHelper

[Chris Müller]

With #64 the possibility to add a node with a multiple type was introduced. It should be also possible to add a multiple type node in a Fluid template.

• A MultipleTypeViewHelper is available, the types can be defined via a types argument.
• Properties can be set with the property view helper or via a properties argument (which holds an array of name/value property pairs).
• The view helpers behaves mostly like other type view helpers: it can be set as property of another type, an id can be assigned and it can be set as main entity of the page.
• The usage is described in the documentation.