Support YAML as Document Format for Rules

[obfischer]

YAML is nowadays a widely used and accepted document format. Therefore we should support YAML based documents for rules.

• Constraints can be specified in a YAML document
• Concepts can be specified in a YAML document
• All YAML documents will be verified against a schema as we do it already with the XML documents containing rules
• Documentation for the YAML format has been written
• Unit and integration tests have been written

[obfischer]

Report support is still missing. There is also still a lot of todo comments in the code.