Remove circular dependency between OdtDocument and OdtCursor.
OdtCursor does not depend on OdtDocument any more. In fact, it is not specific
to ODF any more so at some point it could be renamed.
Make ShadowCursor independent from OdtDocument.
Update EditorSession to the new document API.
Rename ops.Document.getCursors to ops.Document.getMemberIds.
Merge pull request #362 from vandenoever/odtcursor

Remove circular dependency between OdtDocument and OdtCursor.