Within ve.dm.DocumentFragment it makes more sense to call the root node (which is always a document node) a document node, especially since there may be a different node used as a root.
This commit also adds test for getDocumentNode and getNodeFromOffset which uses the offset map.
Change-Id: Ic4609233cedc41f7e5a5f8fdb0e6178652c95554
And fixed ve.dm.DocumentFragment constructor to generate a correct offset map which creates references to branch nodes only
Change-Id: If9e515be0c63d272bfed9bf4da625a48edd36f48
