Add a generic work Item Document

[pellecchialuigi]

To provide more flexibility it should be possible to link to a snippet of the reference document a snippet of another document. That because so far is not possible to create a relation between Code and Test Specification.
It should be possible to handle different versions of the document and to check the effect of changes.
We have to identify a way to propose a view of broken links in such a case.

[pellecchialuigi]

Document work Item support introduced with #37