Fix DocBlocks for Converters

A copy paste error had occurred during development of the Argument,
DocBlock and MethodConverter. The File DocBlock was incorrect. Also the
DocBlockConverter missed a class DocBlock.
Extract Property and Constants from XML Writer

In an effort to refactor the XML Writer to provide better tested and
SOLID code with which bugs can easier be tracked and resolved we
have extracted the property and constant handling into their own
converter classes and added unit tests to verify the behaviour of
those converters.