Map boolean property values to http://schema.org/[True|False]

[brotkrueml]

Currently, the API doesn't allow the setting of a boolean value to a property. But it would be nice, if this is possible and the values of such properties are mapped on http://schema.org/True or http://schema.org/False when embedding markup on the page.

• [x] Boolean values for properties are allowed.
• [x] They are mapped on the according schema.org types http://schema.org/True or http://schema.org/False
• [x] If in a view helper the property value "true" or "false" is used, they are casted to a Boolean.
• [x] The documentation is adjusted.