Fix/issue 93 incorrect tag order

[AlbertWeichselbraun]

• Improved both the HTML and XML Annotation postprocessors.
• Fixed Fix/bug 93 incorrect tag order #94 (XML tags are now closed in the correct order).
• Breaking change
  • the XmlAnnotationProcessor now uses a mandatory root element to ensure that the create XML is valid.
  • Impact:
    1. The created XML will contain a <content> root element, which contains all annotations.
    2. The name of the root element can be overwritten, if the optional root_element parameter is provided to the annotation processor call.
• Documentation fixes.
• Updated dependencies and added pytest as a build dependency.